I would advice to use a gradient for colors under water, red disapears (0-20 mtr) green (0-50 mtr) and bleu at around (0-100) mtr. this means that around 100 mtr complete darkness. This is all relative to the amount of light available at the surface.
The result wil be a much more realistic experience. with natural boundaries which can be eplored with a diving/flash light.
the table i attach is a little more conservative but i understand its stil a game and a little manouverability is wished for.