Honestly? People have tried going down into the ocean before. And they never make it out. You won't really find anything special even if you did go into those ruins, mainly because the sea-peeps would be pissed off enough to destroy most of everything. Basically...right now, the oceans are Cthulu.

Of course, I do have plans in the future regarding the ocean, but for now, it's best not to think about swimming with tentacle monsters.