Water worlds could do fine with thermal vents.
It is just very unlikely that, unless there are physics that don't require heavy industrial engineering, that tech civilization could arise on a water world.
Dan Simmons talks about a pretty cool water world in his "Hyperion" series btw. Benford speculates about a water couple of variations of water borne technical civilixations, but they both involve really really heavy genetic manipulation.
Of course Brin has his "uplift" series with the whales and dolphins, but they are heavily modified by humanity....
I think, unfortunately for the real universe, that Nick is likely right.
Now we just need a young healthy Rhodes, to stand astride the galaxy, from Sol to galactic center.....
The ethics of an interstellar Rhodes will be much less twisted, in the galaxy that seems to be in fact emerging.
Of course Einstein is in the way....
Ed