My response:
Sounds to me like good 'ol hubby is on a "Power Trip", and an irresponsible one at that.
Why is it that your 4 year old can't swim yet? Remember, you have as much responsibility for any death or injury to your child as he would in the event of an incident. There is absolutely no reason for your negligence in bringing that child near any body of water, knowing he can't swim. Completely irresponsible on your part.
It also sound to me like if you "spout off" any type of law to him, that your marriage would be in jeopardy anyway, and he won't listen to reason or the laws. Unless you're willing to get yourself, as well as him, into trouble with the authorities for "Child Endangerment", and take a chance on having your child taken from both of you, I'd keep my mouth shut.
Instead, how about enrolling your child in a swimming class and teach the kid how to swim ? That sounds much more prudent. Also, when your child learns to swim, he'll also learn about safety, and will tell Dad himself that he needs his life jacket, or at the very least, his "water wings".
Good luck to you, and the kid.
IAAL