What type of things can happen to a D&D group adventuring underwater?

They can drown.

Honestly, I avoid underwater adventures most of the time because they add in elements that I don't want to bother with such as the fact that not only is breathing difficult but also weapons might not function as planned and so on. When I want underwater inspiration I re-read the Rebma parts in Nine Princes in Amber. I'm not much help to you, I suppose.

I don't see how the players can beat any native to a strange environment when they are out of their element.  Underwater, underdark, fire or ice plain.  The players are going to have to spend half of their energy just existing.  At best its going to be spiderman vs the hulk, at worst it will be spiderman vs a car jacker.

Depends on the setting and system and how much the PCs can or cant bypass the problem.

Example in AD&D Theres some spells to solve the breathing problem. Without that theres a hard limit to how deep you can go. But there are a few magic items that can allow you to dive deep. Theres also fantasy submarines. Spells cast underwater may have different effects. Such as Lightning bolt acting like a fireball. Movement will be reduced unless you have magic to counter it.

In effect you have to do what people do in the real world. Layer on protections or come up with one big encompasing protection. And that may require research or shopping around or finding a mad inventor or two.

BECMI had a Creature Crucible booklet for playing underwater and underwater races and kingdoms, and undersea adventures. AD&D had some pointers in Wilderness Survival Guide. Also a few dragon articles.

The types of adventures can be all over. Ruins, kingdoms, raids, wars, monsters, invaders, environment, whatever. The main limiter is on what the PCs have access to to allow them to get down there.

If its just holding your breath then theres a very hard limit on the adventures. But theres plenty of danger even just there.
Each new spell or item will extend that range eventually to the point they can move about unhindered.

In Star Frontiers theres an Ares article that adds in underwater rules. A hard limit on how far down you can dive. Limits on vehicles and equipment. And so on.

In Albedo you are limited to just above 2000 tech which means severe limits after a point that their tech simply cant bypass. Considering the setting no one in their right mind is going to try as they are in a way even more vulnerable than space stations. Which are few and far between too.

And so on. Sometimes you just have to say "No" and move on. Or layer on so many handicaps that its not viable.

Quote from: RPGPundit;1009215What type of things can happen to a D&D group adventuring underwater?

Everything that can happen on land except wetter.

A quick glance at 5e shows the following.
2 spells to allow breathing underwater. Alter self which also allows normal movement, and Water breathing which doesnt also grant movement.
At least 8 items and 1 machine. Some just allow swimming, some allow breathing and swimming, and some just allow breathing.

Not including straight up polymorphing.

The DMG has rules for moving at depth but at a glance I see no rules for the actual limit of depth unprotected. Though the rules as stand do more or less cover the effects as prolonged exposure will result in death. If you dont have something that grants a swimming speed then your safe limit is 100ft.

So that tells us a few adventures the PCs can get into. Like deep sea treasure hunting. But they have to get down and get back fast if they dont have the right prep. Both from the limits of holding breath and the depths.

This can also lead to some interesting adventures where the PCs are trapped in an undersea cave and simply cannot reach the surface in time before they drown. They will need to work out some way around this.

In 40 years of gaming with all manner of systems and settings I don't think I've ever had a party spend more time under water than they can handle with one breath.

Well, breathing underwater will almost certainly involve magic, and that means intelligent enemies will have Dispel Magic effects (granted, the 5e drowning rules are extremely generous, but it's still an annoyance).

Even if Dispel Magic isn't an issue, there are indirect hazards. Being swallowed whole, and then having the big fish swim away is a serious concern, even if the swallowed character isn't necessarily dead.

Many underwater adventures have dungeon-like settings. The ol' portcullis trap has an added concern, as simply trapping the characters for a few hours might be as good as death. Falling walls, cave-ins, even a big enough maze might make time a bigger issue than the usual dungeon crawl. Don't forget the ol' Giant Clam clamping down on swimmers, if you've somehow got adventurers not using some sort of water breathing.

Bleeding characters (eg, characters with 10% HP damage) might incur additional wandering monster checks or particularly aggressive monsters, if you're not into hard core realism.

If you're into material components, it's possible such components aren't quite as effective when soggy (same for scrolls, and many ranged weapons simply won't function well underwater).

Beyond the usual sharks, giant octopus, and usual humanoids, don't overlook oceanic electric eels (more like an electric fireball), piranha-stirge swarming fish (with Evasion-like abilities), and tiny poisonous jellyfish as novelties.

Plenty of monsters can be quite easily adapted to the underwater environment.

I had no place in my campaign for Behirs, until I realized that they function much better as a sea serpent. And I changed their electric breath save con-based underwater, with the added effect that failed saves also stun. Wyverns also became aquatic, with vestigal wings that allow them to leap briefly above the waves as well.

Free-floating oozes, slimes and jellies.

Shambling mounds ravaging the coral reefs, and ankhegs, ready to pull the unwary into their dens.

Mimics imitating flotsam or even wrecked vessels.

Sun-maddened myconid sovereign, hidden under a floating kelp raft, with an ever-growing horde of spore servants...

One of the biggest differences to underwater adventuring is that it is a three dimensional environment for everyone. Wear plate armour and you walk on the bottom. Take off the plate and you swim above the walking tank. If you are swimming attacks can come from any direction in the sphere around you.

I love underwater adventures. So many interesting aquatic monsters, new environmental challenges.

I ran an short Earthdawn campaign entirely underwater. The local Citadel had decided that the best way to avoid the horror and horror-constructs was to sumberge their island town into the bay and put up wards where the water got deep. Every citizen who had business outside the pressurized dome of the town was given "blood gills" (a blood charm that resembles an octopus that you attach to your face to breathe). The PCs first had to investigate something wreaking the kelp-farmer's beds. 3-d combat with an Ork Beastmaster and her 3 Giant Sting Rays (aquatic shadowmants - which are flying stingrays on land) and various other sea life (an giant eel, and a barracudda she dominated as she fled their attack) in and among the kelp forest. They found out the ork was Horror-marked and the Horror was using her to distract from it's own attacks on one of the guard towers along the magical ward line at the outer reef. They showed up there and had to deal with what essentially was a corrupt water elemental (2 story tall vortex). They couldn't get hits in and were getting torn to shreds until they retreated to the air bubble at the base of the guard tower and could use some tactics on the thing when it breached into the bubble (as it could only approach from the deep water side. Saved the tower, prevented the fall of the wards, and basically saved the whole citadel from slowly being corrupted via the food supply (which was the Horror's plan once it had breached the wards).

Quote from: Telarus;1009474I ran an short Earthdawn campaign entirely underwater. The local Citadel had decided that the best way to avoid the horror and horror-constructs was to sumberge their island town into the bay and put up wards where the water got deep. Every citizen who had business outside the pressurized dome of the town was given "blood gills" (a blood charm that resembles an octopus that you attach to your face to breathe). The PCs first had to investigate something wreaking the kelp-farmer's beds. 3-d combat with an Ork Beastmaster and her 3 Giant Sting Rays (aquatic shadowmants - which are flying stingrays on land) and various other sea life (an giant eel, and a barracudda she dominated as she fled their attack) in and among the kelp forest. They found out the ork was Horror-marked and the Horror was using her to distract from it's own attacks on one of the guard towers along the magical ward line at the outer reef. They showed up there and had to deal with what essentially was a corrupt water elemental (2 story tall vortex). They couldn't get hits in and were getting torn to shreds until they retreated to the air bubble at the base of the guard tower and could use some tactics on the thing when it breached into the bubble (as it could only approach from the deep water side. Saved the tower, prevented the fall of the wards, and basically saved the whole citadel from slowly being corrupted via the food supply (which was the Horror's plan once it had breached the wards).

I miss Earthdawn.  Such a badass game and setting.

Well there's the bends. Never saw an anti-bends spell, but having that ring of water breathing doesn't seem to help when you PC goes up to the surface after a long stay underwater. Without a spell of hyperbaric compensation your PC can be in for a lot of pain and eventual death. Plus, your PC has to actually breath water after a certain depth or else the pressure crushes the lungs (remember the movie The Abyss?).

Cave diving is a really tough certification to get so I can imagine it's equally tough on novice PCs. Getting your bearings in a practically zero G environment can be really tough and disorienting and then there's the whole thermocline and halocline issue. Haloclines are an issue in caverns in the Yucatan peninsula where salt and fresh water meet. This is an example of a halocline:

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The water density difference acts as a mirror making it impossible to see into the other side, behind which all sorts of evils can lurk! Without a means to communicate a party member can be right on top of you and yet invisible to the other members of the group.

Characters with armour may need a sacrificial anode or galvanic anode that corrodes faster than their armour and weapons so they don't just rust away during the adventure. BTW, a sacrificial anode spell will sufice, but depending on region they might have to research it prior to adventuring.

Drinking and eating underwater can be an issue, specially in salt water environments. How do you open your waterskin without all that seawater rushing in and spoiling it. More so, will all that salt water in your lungs (due to deep diving, see The Abyss above) do some sort of inverse osmosis and dehydrate you from the inside out?

Oh and then there's the pipi and the popo. It would be a real drag if the wizard who built the underwater dungeon put a urine-tinting spell on those 50' of corridor you need to sneak through. BTW, as mentioned before, anything drifts a long way under water, specially blood.

Heck and I haven't even gotten to the monsters and traps. Did I mention how crushing an explosion under water is? That's why a depth charge is such an effective weapon against subs.

Run into a singing crab and participate in a musical number regarding how much better things are below the waves?

In 5e they handle it thus.
If something grants a swimming speed then it also grants protection from the depths.
If all it grants is water breathing then it does not protect from the depths and anything past 100ft becomes dangerous over time.

Not exactly realistic. But it gets the job done with the fewest rules. and if you want more detail or realism feel free to add it yourself.