Karlyn Pipes-Neilsen has a video on that subject. You will look and feel like a spaz, but who cares, you're not there to impress anyone.
https://www.youtube.com/watch?v=a1KIDbO9Gdo
If you can't enter the water to warm-up, you'll have to do dryland jumping jacks, box jumps, or running in place to get your heart rate up.