Light and Color
Underwater, things like fish appear 1 and 1/3 times larger and ¾ times closer due to light refraction as light enters the mask. As a result, when reaching out for something like a rope which will appear larger and closer than it really is, you should be careful not to misjudge distances.
As well, the deeper the water, the more light is absorbed, so first red, then orange, yellow, green, and finally, blue become more drab and hard to distinguish. However, use of a dive light will bring to light all the stunning colors of the ocean.