Hidden Worlds of Warcraft

[TheLuminaire]

There's that one in the NE lands, with the Hydra at the end of the level. Filled with Naga and Turtles. I forget the name if it at the moment, but it's a common, low-to-mid level instance. Very popular, and very fun, too.
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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.