I was taught to be comfortable in the water at the age of around 4 or 5 months of age. My father drowned at the age of 6 after diving off the end of a pier in shallow water and hit his head on a rock. A crabber walking along the shoreline seagrass with a dip net looking for blue crabs stepped on him and found him got him up and managed to resuscitate him in time.
After his discharge from the Navy at the end of WWII (In the Asian Theater of war) he spent time as a Navy Medic on Okinawa after the Invasion Landing (of which he was a part of). He learned the Japanese/Okinawan method of teaching infants/toddlers to get familiarized with the water as Mal mentioned in his post above. I was born in 1947 and my dad used the same method. At 4 or 5 months of age my father took me to the pool and/or river and would put me in the water (just holding me in the water until the shock factor wore off and then would just release me and let me sink to about 2 or 3 feet down at which point I would just instinctively right myself and swim to the surface. Dad told me when I was old enough to understand that a baby automatically feels instinctively how to right themselves and hold their breath and swim for the surface due to spending the first 9 months of their lives fully immersed in the water. As soon as the back of their head breaks the surface you need to lift them up as they don't have enough strength to lift their heads up to get air yet.
I've been a swimmer all my life because of that, to the point I gave up a high paying job in Washington DC as a Tax Accountant to become a deep sea diver. In fact that is how I eventually ended up here in Singapore with Oceaneering International way back in 1982.
After my son was born, at about 5 months of age I took him to my friends
condo here in Singapore and did the same with him (friend was also a Yank and also one of my diving crew). My son was never traumatized nor did he cry at all. (Only his mother) Today, some 37 years later, he is still a avid swimmer. But I have to say my wife (local) was freaking out poolside as she is deathly afraid of the water. Fortunately my friend's wife (who is also local) knew what was coming and managed to calm her down).
Just to confirm what I'm talking about this is one of the pics taken that afternoon with my son back in 1989
.