It's a big old world down there, and one hardly touched upon in fantasy gaming. Sure, there might be a section or two discussing it and the odd adventure that requires water breathing or the use of a potion or two.

If you wanted to set a whole campaign under the waves, here are some things that i think you need to address, whether you are a native or not.

1) Tides & Currents vs Movement - More specifically near the coast, the movement of the water around you has a bit more of a bearing on your ability to manoeurvre than, say, the wind, on land. Just swimming about can be at the mercy of prevailing conditions.

2) Encumbrance vs Buoyancy - If you're an adventurer geared up and ready for an underwater adventure, your gear is going to weigh you down, adversely affecting your ability to use the 3d environment of the deeps. Not weighed down and your ability is similarly affected, except in the upper reaches of the water, instead of the bottom.

3) Thrusting weapons vs Other - The resistance of the water means that swinging attacks or using stuff that isn't streamlined is a bum rap. Short range grappling and using of small thrusting weapons is likely to be most affective.

4) Spells vs The Environment - Big one this. How do typical spells used on dry land work under the waves?

5) Speech vs The Environment - Unless you're a whale or something able to use the sonic condusiveness of the water, you're screwed on the communication front unless you have telepathy or can utilise sign language.

6) Seeing Distances vs The Environment - Tide, debris, the depth at which you are operating all have an effect on your ability to see. The 3d nature of the environment also means that you have to look up and down as well as outwards.

Any other considerations worth contemplating?

How do you tackle the ones already mentioned and any more that are raised?

I've always wanted to run a series of adventures underwater.  For the effects of the new terrain this I'd go the Unknown Armies route.  If negative conditions effect all combatants then there's no need to apply any penalties since everyone is on equal footing in that respect.  Anyone with a leg above the competition would get bonuses.  A flying spell would give fine control over maneuverability for example.

The big thing about encumberance is just to decide how much a character can carry before they lose the ability to swim and just be stuck walking on the bottom.  Most games have mechanics for swimming that would work nicely here.

Defense will be hampered too, if your game uses any amount of passive defense it might be ok to just give a reasonable bonus to thrusting weapons or nets; and ignore penalties to other equipment.  Hell I wonder how easy it is to use a shield effectively under water.  Again if most everything is on an even footing, penalties are just unnecessary extra steps in encounters.

For spells there are two options.  In DnD 3.5 I seem to remember them saying that all spells work without change underwater, the magic overrides any natural warping of a spells normal effects.  The more fun option for me would be to have spells change sensibly with the new terrain.  Lightning Bolt in 2nd Edition ADnD used to become a sphere if cast in water didn't it, or was that an optional thing?

Magic items are also a consideration.  Does a shock weapon cause any undue effects on adjacent allies and the weilder when it's activated, or does the magic contain the effect?

I'm currently building up my legions of underwater minis in preparation for a lengthy underwater adventure, which I've always wanted to do.

EDIT:  The lightning bolt turning into a sphere thing always kind of bugged me.

You go underwater, and suddenly something resembling physics reasserts itself?  Pish.

I think it's more interesting to create new spells for primarily underwater casters.

Like a locathah sorcerer whose primary attack spell is "Piranha Current", a spell that transforms an area of water into whirling frenzy of "piranhas" created from the salt in the water.

Quote from: Jackalope;258483EDIT:  The lightning bolt turning into a sphere thing always kind of bugged me.

Yeah, I don't know the science of what lightning would do in water.  I understand though that salt water is highly conductive and might be the reasoning behind that old rule.

Quote from: Jackalope;258483I think it's more interesting to create new spells for primarily underwater casters.

Like a locathah sorcerer whose primary attack spell is "Piranha Current", a spell that transforms an area of water into whirling frenzy of "piranhas" created from the salt in the water.

Yeah, that's the kind of stuff that interests me.

Shadowrun, owing to its renewed interest in underwater arcologies, contains extensive rules for underwater gaming, including visibility, spell effects, movement, and so on. They're pretty good, and include extra gear for things like releasing ink clouds, and minitorpedos and gyrojets and so on. Adapting such things to a fantasy environment with utterly fantastic elements, however, would be difficult, given that the SR underwater rules do their best to be realistic, which could be jarring for those whose fantasy rules are utterly fantastic. [I wouldn't be playing those games, anyway, so it doesn't bother me!]