Well this is an idea I've thought of, not sure if it's been done before. Basically I'm asking for a mod that lets you breathe underwater, as that would open up the whole sea to explore.
However it may be a bit OP, so how about a craftable item that merely EXTENDS the time you can be underwater (by a lot), but so you still have to surface eventually. I think these would be great mods that would open up the mine craft world in a new way, and allow underwater building in Survival. While not nessesary it would be nice if it worked in Multiplayer as well.
I've recently found that there is another way to breath underwater if you have IC2 installed. The Quantum helmet, when fully changed allows you to breath underwater - expensive in resources though ,,,,,,,,,,,,