Tim,
There is very nice snorcling around the island incl Gouvernor , you just have to go on the left or right side of the bay when it's calm sea , close to the rocks and you will find great places to see fish and turtles . You need to be a good swimmer with fins and should only go out in calm weather conditions . Great spots are : Reef of Grand cul de sac , Gouvernor on it's outer left and right side, Columbier sides and island , Lorient reef , Gros ilets shipwreck and around island ,Shell beach left side around the rocks , Saline rock in the middle , Petit anse in calm weather only ,
Fourche island and Toc vers . You will find lot's of turtles ,ray's ,barracuda, smal sharks, and huge swarms of tiny colored fish .
Just be careful with the weather ,boats , be in good physical conditions and wear fins if you do longer trips and you will find a paradise under the water.