Now that Arma3 Alpha was updated so that you are unable to throw grenades under water maybe there is a chance to still be able to use the "G" key. "G" key could be used for anything really, but my contribution of ideas for its use would be an under water melee, with your diving knife which every scuba diver have.

This could be useful in many ways when sneaking up on enemies in the water, or if ever added/modded, defend yourself against sharks(referring to the crocodile mod in Arma2).