Already been done, there's a fleet of them in the Pacific (and other oceans) mapping seafloor and isotherms as we speak. They operate on batteries, go 1/2 MPH and save energy by using fins to porpoise their way around. They have positive buoyancy so when their batteries get low they rise to the surface and transmit position.
BUMP
Glad to hear we have such a program. Guess we refer back to an earlier post, sit and contemplate a good map, or better a globe, They be one hell of a lot o' water out there. "Here there be dragons" probably ain't far off for some of it.