I used to take pre school DS1 swimming with me, and one half of the pool would often be sectioned off for swimming lessons. My DS would be messing about, playing, having fun etc, and he used to look at the kids being shouted at to swim up and down and say he never wanted lessons because it didn't look much fun.
Then along came DS2, and being a single parent I have found it hard to take them both swimming, so it doesn't happen very often.
DS1 goes swimming at school now, and sadly he finds himself surrounded by kids who've been having swimming lessons since they were toddlers. So at 7 he's the worst swimmer in his class, which makes him sad.
I try to take him when I can, but it's hard to fit it in with everything else, and it spoils the fun if we're having to do it rather than going because we just want to.
I hate the culture of formalising activities that should just be fun. When I was 7 and we went swimming with school, none of us had had lessons, so we all swam with floats and had fun and felt equal. There seems to be an obsession these days with pushing kids so that they're ahead of their peers from a early age. I remember being told to send my child to a particular pre school because "they'll teach him to read before he starts school, so he'll be ahead of all the others in reception"!
I think water confidence is important, but I don't think it needs to be in a formal setting at such a young age.