Hi
If you are shooting RAW, then you can set "cloudy" white balance, and sort it all out in Aperture or Lightroom, however I don't think RAW file format is an option on the Finepix F50, so it is probably best to shoot WB manually. I believe that this is good practice, and it is easier eventually to get it right on the camera first, anyway.
I have in the past set WB to cloudy and left the WB alone, and balanced on a slate at arms length, but currently I WB each shot.
WB on a patch of reef near you subject, take the photo, review the picture. If not the correct colours, repeat with a different patch of reef. (By far the most fiddly, but best results for me - if the picture is too red, select a pink reef patch to WB on etc..)
You really need to search the manual for your camera as well, in order to set a user definable button to carry out WB, so that balancing is quick to do rather than flicking through fiddly menus.
Cheers
Hal
Check out these other White Balance threads
1 , 2 , 3