A slave flash is the best for this if your using a compact, thus allowing your built in flash to trigger the slave, the benifit to the slave over your built in is its away from the camera and usually you can angle it where you want it. as previoulsy stated the biggest problem is the back scatter you get in the water , this is generally caused by plankton, and the bigger the bloom the worse the back scatter is. i dived Lundy this weekend with the seals and the plankton bloom was so bad it was totally pointless taking anything, even up close with macro you get terrible backscatter. as for the med youre going to have to deal with the very blue water, so if you don't use a good flash all your photos will just come out blue, a dissapointing result, but again that depends where in the med you go. my photos from Spain come out a completely different colour to those around Italy and Sardinia.
hope this helps
Martyn