We had a friend, a professional boat captain, who made his young son wear a life jacket whenever he was near the water - on the dock, whatever. His comment to us was that, since he had never known any other way of being near the water, he didn't argue or complain or find it restricting.
It's my opinion that consistent and strictly followed rules are readily accepted by children. It's the inconsistent, sometimes followed, sometimes not, that cause problems.
We have twin grandsons, and as I'm sure you know two of them is more than double the work. I think it's a wonderful experience for them, but you will have to work on maintaining a sense of humor occasionally.