Although I can not think of a way to fully protect a cube underwater, a good solution might be to instead purchase a few YJ Guanlongs as they are very cheap. I would imagine the problem with almost any cube is that water would seep into the gaps between the pieces that form cubies, so disassembly would most likely be required (of cubies at least) to dry it out. Lubricating the core of a cube with silicon lube might offer some protection to metal parts (if you use the high viscosity stuff). But I imagine over time rust will set in on any cube, hence why Guanlongs might be a good choice due to price. I myself have bought a few Guanlongs with the intention of underwater cubing, but haven't got round to trying them underwater yet, (the cubes themselves perform pretty decently). Cubezz sells Guanlongs at very low prices (I got mine from them). I hope this is of some help to you.
