We love Coki Beach. You can rent equipment for 5 dollars... no need for fins, and you can see fish while you're standing in shallow water.
You can also feed the fish with dog bones they sell at the beach.
It not as pristine as St. John, but the 'beach people' are harmless.
Trunk Bay is beautiful, but when you start multiplying the transportation and beach admission by 4 it adds up quickly.