We visited 3Sail church during one of our trips. It's one of my favorite memories of St. John. Puddlejumper is a forum member and who invited us to attend. I know the location has changed since we worshipped with them, but if you PM him here I bet he'll respond with info!
I was on STJ in March and attended a church service on the beach at Cinnamon Bay. It was an amazing experience and I highly recommend it. I found out about it via a notice on the Cinnamon Bay bulletin board.