I alternate between holding him and using a flotation device. Ive always used something like this with all of my kids. I think it helped them learn to swim bc they were able to maneuver themselves in water. My oldest kids are 4 and 6. They both know how to swim. Its definitely not a substitute for learning to swim or adult supervision.
My son is 20 mo and is currently in swim lessons and I was told if you plan to get a child into swim lessons, they prefer kids to never be vertical (meaning legs dangling) in water in floaties. Only put them on floats where the legs are fully on it or being held by family. Once the child has gone through lessons they show the parents how to work with the child in the water but until they can swim this is the best thing you can do for your child. It definitely makes it tougher on families but it is safest! This is just what I was told, so just use your best judgement and always make sure you are close by.