I've been doing a lot of swimming at the pool with my boys, and I've discovered something fundamental: If you have a child who is uncertain in the water, buy them goggles.
I always assumed that breathing was the thing that put them off, but that's the easy part. They don't like not being able to see, and the goggles are a cool toy that are only really fun if you are underwater. I have two boys who needed help finding their confidence in the pool, and goggles worked for both of them.
It's been a real pleasure seeing my guys become the type of kids that just throw themselves into the pool, flailing and gasping and loving every minute of it.