Before you can swim you'll need to take off armor and other heavy items.

 

Search through the sand or reefs to find coral and oysters.

 

Beware though....don't run out of air!  Also watch your fatigue or you'll become too tired to move.

 

To check the oyster you need to have a knife or dagger in one hand and the oyster in the other and OPEN OYSTER WITH KNIFE.