It's very similar to Escape from Alcatraz, jump off the ferry and go. Practice warming up in place in a 2' x 2' space (on the boat), high knees, jacks, stretch, etc. Bring up core temp, drink fav sports drink room temp or warm in thermos - keep core warm. Put wetsuit on (if wearing one) no earlier than 30 mins before race start. And warm up with wetsuit on too. Jump in and you will be ready to go.
Here's a good article from Bonnie Ford, ESPN, and part of the discussion is about Escape from Alcatraz and other races where there is no swim warmup before the race, i.e. boat start race: Are triathlon swims getting safer?
Good luck an have fun!