I like this idea already. Powers hmmm, lets see. Would you approve of any of these, I am still trying to pick one.
Weather manipulation or atmokinesis?Good for battle and sailing.
Animal mimicry?Plenty of animals underwater to mimic the abilities of and would help a great deal.
A limited form of molecule manipulation?Good for combat and building.
Telekinetic energy generation?Useful underwater to remain capable.
Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn