I don't have an expert opinion but if it were me personally, I would get the one with the crotch strap. The problem, I think, when you don't have the crotch strap is that, theoretically, he could just slip out the bottom (imagine how it looks when you are taking a shirt off and his arms are raised). I wouldn't be as concerned if I was using it in a pool, but the ocean is a whole different deal. Even if it fits him snugly normally, a strong wave could pull it off.
Also, speaking from personal experience as a small person, the crotch strap prevents the life jacket from riding up and constantly being up around your ears which is really not comfortable.
Again, I'm no expert so take my advice with a grain of salt.
My DS has impulse issues too so I hear ya on the running right into the ocean thing! We went to the park today and my dad wanted to sit by the river!!! Uhhhh...not if I actually want to sit down while I eat!!!
Have fun in Florida.