Oh sorry, I thought you meant an underwater entrance and then you have to resurface, while here the entrance is just a hole at the bottom of the lake :V
However, there's not much to see except the waterfall and lavafall, because it's just a hole, nothing to explore... (EDIT: Scratch that, there are four sort of ravines meeting at some point down there, good mining opportunity)
An other interesting thing I noticed was a mountain cave, I mean the entrance was impossible to reach by foot (some 70 blocks high).
EDIT: Gotta edit this again. Cave networks are simply amazing now. You should try this seed: 2015284918655602569
underwater huge cave network entrance at X -494, Z 211 (and pit-like entrance at -463, 150 and -441, 138)
And I keep discovering new entrances in the nearby mountains. Only one thing to say: don't get lost

Oh, and the underwater cave is at -72, 258.