Predating the LaChLuVe fish is this Air Jelly made by Festo, a German automation company. The unifying goal of these creations is to achieve neutral buoyancy. From there, propulsion becomes as effortless as it would in zero gravity. This jelly uses helium for buoyancy amd when deflated, weighs in at 1.3 kG. A Li-poly battery as its only power source, and a series of gears, shafts, and cranks power the arms. It can be steered by control of a pendulum on the underside.
1.3kg gets you a lot more to work with than it used to.