I have done lots of snorkelling as well as diving in the Maldives. I would not worry about a strobe, you will get close enough to the fish and corals (and fingers crossed other things) not need one in the clear shallow waters of the reefs edge. One thing I would say is try and take pictures in shallow water so you are letting lots of ambient light in. There is a LOT of life at 2m and under.
If I remember rightly the instructions of the sea and sea strobes at least the YS D1 have a guide of setting it up the correct way by pointing it at the camera and taking a picture..... If you see the flash it the picture it will be firing at the right time.