Hidden Worlds of Warcraft

[Roland]

Blackfathom Depths is probably the one you're thinking of.  It's been a while, but last time I was there, there's only a room or two where you even have to swim.
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Yes, that is most definitely the one. Thank you. :D And your memory isn't too far off. It does not have a huge amount of under-water content, but it does take place in a cave, with watery depths that you must swim past. And, of course, the Hydra battle takes place in shin-deep water.