- Underwater pit traps. Sometimes the only way to get from one place in a dungeon to another is by wading through an underground stream. Rarely, though, do the PCs think to pull out those 10' poles while doing so. This will cure them of that omission. It won't do any real damage, except getting wet anything they might have been holding over their heads to keep dry, as they plunge into a sudden 10' pit. Those wearing heavy-duty armor might be in for a bit of a shock as well. Swim? In that?
- Water-only access. There could be entire areas of a dungeon that are accessible only by going underwater. If the PCs demure because of the notion of having to hold their breath, or because they want to hold on to their potions of water breathing, then they lose out on the goodies.
- That's not water... Speaking of potions of water breathing, and all the other various and sundry magics that allow for relatively easy action beneath the surface, what if the PCs encounter an area that is filled with, say, oil? Or an underwater pit that is filled with some heavier-than-water substance (so it sinks, filling the pit) that will foil such magics and yet leads to some place of interest?
- Oxy-gum. The cheesy 1960's Japanese cartoon Marine Boy had something called oxy-gum. You pop it in your mouth and it turns the seawater into breathable air for a time. If you're looking for a non-magical way to let your PCs delve into the depths, you might give them access to a plant or herb that has the same effect, for a limited time.
- On a (Not So) Slow Boat to Adventure. Nobody says the PCs can't travel on subterranean rivers in style. They come upon a fancy paddleboat, crewed by skeletons, or permanent unseen servants, or orange snirfneblin, or what have you. They pay a coin for passage and climb aboard. What could possibly go wrong?
There's no earthly way of knowing
Which direction we are going
There's no knowing where we're rowing
Or which way the river's flowing
Is it raining? Is it snowing? Is a hurricane a'blowing?
Not a speck of light is showing
So the danger must be growing
Are the fires of hell a'glowing?
Is the grisly reaper mowing?
The danger must be growing
For the rowers keep on rowing
And they're certainly not showing
Any signs that they are slowing!
- Three-Way Fountain. The PCs come across a three-sided pillar deep in the dungeon. Each side has a different face, from whose mouth flows a stream of colored water into its own basin, red, blue, and yellow. Alone, they are mildly poisonous or mildly curative (1 h.p. damage), determined randomly with each drink. However, once you start mixing the streams, you are able to create potions with magical effects; two reds and a blue make a potion of speed, two blues and a yellow make a potion of extra healing, etc. Perhaps the PCs find a key to some of the recipes somewhere else in the dungeon. Lest the PCs think they can just plop down here and go into business for themselves, have the potions lose their magical effect after 24 hours. And naturally some combinations will result in explosions, poison gas, etc, just to add some zest.
- Scrying pool. The PCs come upon a deep black pool (or well). Drop a gem of not less than 100 g.p. value into the water and the ripples will function as a crystal ball for a round or two. Make sure it's in a place that's somewhat difficult to get to, or the PCs will be dropping gems into the thing like dimes in a gumball machine.
- Flooded Level. More than just having a water feature, you can have an entire level (or more) of your dungeon completely flooded. Bring out the water trolls, nixies, giant pike, and so forth. This gives you the opportunity to trot out those underwater adventuring rules from the DMG without having to leave the comfort of your own dungeon. It usually works better if you hand-wave the need to breathe with oxy-gum, giant conch-shell diving helmets, piles of bubbles on the dungeon floor that randomly release large bubbles of breathable air, etc., but of course you can build real tension by putting a time-limit on when the air runs out. Plop a kitchen timer on the table and say; "You've got this long before you drown. What are your characters doing?" Lots of portcullises are a bonus in such a situation.
- Hollow Man. If you regret ever letting that certain character find that ring of invisibility, here's your chance to make things right. If in a shallow pool or stream, all the monsters need to do is aim above the two foot-shaped indentations in the water. Or, if under water, aim at the big person-shaped space where the water isn't.
- Wave pool. The PCs encounter a rather large cave with jagged rocks lining the shore. A largish underground lake is there. Every few rounds, however, a huge wave crashes onto the shore, tossing anything on it against said jagged (and painful) rocks. Perseverance would allow the PCs to discover the cause of the waves (some sort of large stone piston beneath the water), and enter into a treasure chamber (via a secret door in the piston chamber, only accessible while the piston is resetting to slam down again, and so very dangerous to get to).
A bit of horror from my childhood - Back when I was a kid, I couldn't get enough of horror movies. We're talking like 1975 and earlier horror movies. So no Halloween, Friday the 13th, or any...