Red filter?

[dbuster]

Just bought my first, relatively inexpensive, digital camera with underwater housing and plan to use it on an upcoming Caribbean trip. I know that, below about 20 feet, I'll need a filter to compensate for the lack of red light in the spectrum below that depth. I think I need a red filter, but I'm not absolutely certain. And is there a particular NUMBER of red filter I need?

And, just for a point of information, if I'm at, say, 50 feet and am shooting up towards the surface (maybe I'm shooting another diver who's a couple dozen feet above me or some such), do I need to keep the filter on the camera? I'm guessing yes, because I'm still shooting through 50 feet of water, but I'm uncertain.

Thanks much.