there is a cenote (freshwater river)that empties (underwater in about 20' deep)into the bay at puerto morales (ask the dive shop there, they can tell you exactly where to park, enter and snorkle) it's not TOO hard to find, you can feel the cold water when you get within 30 yards or so. neat stuff around that hole, including eagle rays and resident squid. it's a decent snorkle, with several nice large coral heads...
that being said, it does not compare to akumal for snorkling. we saw rays, turtle, tons of fish, 'cuda , permit and on and on and on...
depending on how good of a swimmer (daring?) you are , you can snorkle to where the pangas(boats) come in and out of akumal bay. stay on the right side (facing the ocean) of the channel (you can't miss the channel, it's deeper, with no coral)(( WATCH FOR BOATS)) anyway, when you get out near the bay entrance, in the rocks there are several caverns at about 10' deep that sharks sleep in. really easy to see once you get there...
if you can only do 1 spot, do akumal. if you can do both, hit morales and check the "hole"... most of all --- have fun!
snorkling AT playa is pretty non-existant...