I've been there more than several times. the reefs are pretty beat up but if you are blown out of other areas it is usually a good place to get into the water. I've always had good luck finding turtles in the grassy areas (or they could be anywhere) by just following the buoys out that are along the right side of the boats as seen from shore. Follow this line out near the reef. If there is a snorkel tour boat there, you could just watch them because as soon as their guide spots a turtle it's like a stampede of the tourists to get a view. The designated swimming area is to the right of the buoys but if you get between the reef and where the boats are parked on the left side of the buoys you will actually have better corals and more fish to see. If you get way out into the boat channel and beyond the reef, you will be in about 20-30' of water, a smooth, slightly contouring rock bottom. I did see a nice eagle ray cruising there once. Not much current but that can change anytime.
yal ku, near there, has really become a cesspool and I don't think I'll go there again.. there is so much friggen toilet paper and other garbage floating around if you go out towards the ocean, it's really sad. I would imagine the water quality all around akumal isn't much better.
oh yes, akumal bay is shallow.. to the reef max is maybe 10'.
