At the moment I dont know what white balance is exactly. But I do know it is important and helps make your shots look great. The good news is that you can alter it later. Shoot in raw and then expect to do this or manually set it with a white slate or diving tank as the 'test area' at differing depths.

here are some links from people that know more than me 

http://www.uwphotographyguide.com/underwater-photography-ambient-light-manual-white-balance

http://photoblogstop.com/photoshop/accurate-white-balance-adjustments-in-photoshop