Helium balloons stay aloft with no effort, but they’re impossible to steer and easily buffeted by strong winds. Airplanes have marvelous control, but they run out of gas after a few hours. It would be ideal if you could combine in one vehicle the lighter-than-air benefits of a balloon and the controllable aspects of an airplane. No one knows how to do this in the air, but underwater it’s not so hard.
The result is essentially a cross between a helium balloon and a glider. By varying the density of such a vehicle the way a submarine does, you can make it climb or sink. This climbing or sinking motion gives you enough velocity to use your underwater “wings” to generate forward motion, and you’re off to the races. If you constantly vary your depth in a sort of sine wave profile, you can swim huge distances for very little effort.
This is exactly how the diminuitive and unmanned Spray (named after famed solo circumnavigator Joshua Slocum‘s little boat) swam across the Gulf Stream from Boston to Bermuda: Autonomous Robot To Cross the Gulf Stream Underwater. It was a slow, meandering process (check out the plots at the bottom of the Spray project homepage) but it worked, tirelessly and continuously gathering temperature and current data cheaply and where it could not otherwise be had.
Presumably you could do the same thing with a human on board, but it would sure be a slow, uncomfortable ride. On the other hand, I don’t see why you couldn’t ship certain kinds of freight very slowly and cheaply this way.