The RX100 does extremely well in low light. Most of the time you will probably be shooting at the wide angle end of the zoom range where the aperture is f/1.8, so there should be no need at all for flash at snorkeling depths. I have used several small Canon cameras in UW housings with very good results. I have tried the on camera flash on a few occasions, but it did not help much.
I have the RX100 now and hope to get a housing when the prices come down.