This is a set of pictures from London Baby Swim, a school where they teach babies as young as 6 six weeks old to swim and to develop their motor skills. Of course, the parents take part in this process :)

As you can see, babies are natural swimmers and can be underwater without a problem. The thing is that “Up until the age of around nine months babies have a miraculous gag reflex which blocks off their windpipes as soon as they are underwater, allowing them to instinctively hold their breath,” explains Phil Shaw, the founder of London Baby Swim.
So, the sooner you put your baby into the water to learn it to swim the better.