Sounds like you have already been given some great suggestions and thoughts, but I just wanted to add a few things. First, my parents have a boathouse(a little different, but similar concept) and we spent most of the 4th out there and my son wore his lifevest all day WITHOUT COMPLAINT. I tell you this not to brag, but to hopefully allay your fears - this might not be a complete disaster. We just have a very basic ($10) life jacket, CG approved, that even has a leg strap and he LOVES wearing it. In fact I took him inside and took it off to 'give him a break' and he insisted on putting it back on. For whatever reason, he likes it. To encourage this in your son, maybe buy it now and have him wear it around the house or in the yard to play - make some sort of game of it. Call it 'boat practice' or whatever, anyway, that will show you a) if maybe he might even like it (so you'll feel better) and b) that the style/model you have is comfortable for him, or conversely, does not work for him at all. Better to know these things BEFORE the trip, right?
Anyway, good luck! You will have to be diligent about keeping an eye on him, but I was SHOCKED by how comfy my son was in his jacket (he is only 22 months), so the trip might just go better than you expect!