Get the Magic Filter. They have two versions.
The original filter is great if you don't mind calibrating the white balance manually. Some people wear a white glove. Others use the sandy bottom or a slate.
The newer Auto Magic Filter is designed to work with your camera's automatic white balance (don't use the underwater setting). It would be the easiest with the F50. I would use Auto ISO 400. That will allow the camera a fast enough shutter speed to freeze motion. You may wish to use some exposure compensation. From my experience, I usually set it to - 1/3 or - 2/3.
In practical use, the Magic Filter allows you to maintain nice reds and oranges, while still keeping a blue background. I think it's far more effective than just using the camera's underwater mode only.
Filter photography is usually done in shallow waters, without a strobe.
URPro filters are said to be similar. I haven't used one.
I put up a gallery last year. All of these were taken with filters.
http://www.pbase.com/krancer/fiji_2006_