Have you seen this one?
Critters Inflatable, Pet Preserver, Pet Life Preserver, Pet Life Jacket, Product Page Not very bulky plus it looks cool.
It's been my experience that anything you try to attach to a cat, even a collar, takes some getting used to. Give him some time and he'll probably accept it.
BTW, I've heard that with a cat on board, when tied up you always hang a towel over the side touching the water, so he can climb back aboard if he misses that seagull he was trying to pounce on.