Everyone says it's good at Xtabi and Samsara, but I didn't see much sea life out there last reach .... I hear good things about Catcha, though there may be issues there about using the facilities if you're not a guest of the hotel ..... There are lots of little boats that pick u up from cliffs and take you to a reef, which is usually better than right off the shore .... But some say they see a lot of stuff out there right near the rocks, so it might have to do with the time of day or other factors.