If you're shooting RAW it doesn't make any difference, the colour space is only assigned when the shot is converted to another format (JPEG, TIFF, PSD, etc). This setting only affects the colour space used for JPEG's. If you're shooting JPEG's and going to print from them set the colour space to Adobe RGB, if only displaying on the web use sRGB
FWIW I set my camera to Adobe RGB just in case I ever get the urge to shoot JPEGs.
explains the difference in an easy way. If you want to make high quality prints it is best to work in AdobeRGB but there are some traps in the work flow from capturing to print. For example, most photo printing places won't actually supports anything but sRGB, your monitor can't display all the ADOBE range so you have to guess a bit etc.
Before you start to shoot AdobeRGB, do some tests all the way to print and make sure you get what you think so you don't make a mistake wit those photos you will never be able to recreate. sRGB is really an easier work flow from start to finish (less surprises). For underwater shots though, use RAW so you have maximum freedom to change whitebalance etc. afterwards, introducing as little artifacts as possible.
Finally, in order to really make use of AdobeRGB, you need a good printer and you need to be able to calibrate your monitor so you have a calibrated workflow from start to end.