I try to warmup a little if I get to the race site early enough. I warm up by simply getting in the lake and swimming. It's good to get used to the water anyways in case it is exceptionally hot or cold. I always swim better in the pool or OW after I've already done several hundred yards at least. I don't swim hard during my warmup or real long, just nice and easy to get the shoulders loosened up and ready to go. Usually 5-8 minutes tops.
I like to do a 5 min jog with a few sprints, just enough to break a light sweat, then head down to the water and swim out to the closet bouy and back. Gets the blood away from my organs and into my muscles so they're primed for when i need them.