A:
It is true that red is the first color to change but it does not become invisible. It's because of the wave length of red light. Sorry, but I promise that's all the physics I'll put into this answer. Basically red light is the weakest of the colors, as light travels through water it gets weaker which is why at depth there is no light at all. If I remember correctly red doesn't "disappear" until around 80 feet, and even then it just turns greenish. Hah, found that out on a wreck dive about 120 feet down when I cut my hand. Haha, there's nothing like bleeding green!