T.M.
I teach swimming lessons (parent/child classes at 6 months and lessons starting at 2 and a half) and I love the speedo swim vests with the strap through the legs. I don't use them in the actual lessons, but I allow them for the parent/child classes. My 2 boys had very buoyant bottoms and this vest really helped them (and my little one, who really doesn't need a float, still uses it when he is playing in the pool).
Another parent in one of my classes swore by the suits with the built in floats. The floats in his suits needed to be filled with air. They made me nervous because they could possibly pop.
The main thing I tell the parents in my classes is to push their bottoms down. When their legs float up, their chins go in the water and this freaks many children out. The more often you push their bottoms down and tell them to run their legs, the more independent they will become.
I hope this helps,
T.