In a recent adventure, my players donned primitive diving suits and braved the depths so that they might confront the villainous merman Seldak. D&D has some very simple rules for underwater combat and movement, but on the whole I didn't find those rules particularly dangerous. So, here's what my players had to contend with.
First of all, at the beginning of every turn, they had to make two checks. One was an Equipment Check, to make sure their diving suit and breathing apparatus didn't malfunction. The second was an endurance check against water pressure damage.
The DC for these checks changed depending on what "level" of water their character occupied; at the very bottom, the checks were very tough, while closer to the surface, they were easier.
If the endurance check failed, they took water pressure damage-- said damage, again, increased with the depth of water they occupied, ranging from 1d6 to 2d12. If the first equipment check failed, the endurance checks became harder. If the second failed, they took that level of pressure damage. The third equipment failure gave them no air to breathe; then they started making endurance checks against drowning, as per the standard rules.
If you fail a swim check by a certain amount in the vanilla D&D 4E rules, your character not only forfeits their movement but sinks a square. With my water pressure rules, you also instantly take that level of water pressure damage.
Players can swim freely up or down, but if they move more than one level up or down at a time, they get the bends-- double the water pressure damage followed by ongoing standard water pressure damage each turn, save ends.
This probably all sounds a little extreme, and, well, it was. It made for a much more dangerous and memorable encounter, one they got through just barely.
I wouldn't recommend these sort of rules for everyone-- it slowed the combat down a bit and could be pretty unforgiving.