Lucy,
Here in Melbourne there is a woman who starts babies swimming basically from birth. She advocates starting them very early as you can easily teach them to take a breath before they go under the water, whereas once they get to about 10 months it is difficult to teach them that again. (apparetnly they have it naturally when really young) I tried my dd at 5 months and she hated it and screamed the whole time but the other 7 babies there loved it. They can be very confident and in theory, can get themselves out of the water if they fall in by the time they are 2.
I think it is a great idea especially in a warm climate where you are.