Proper lessons could bring them on a lot, Nikki, although you might want to hold off until they are 7-8yo to get the most for your money. Do they not get swim lessons with the school as they get older?
I said this elsewhere today; DS was a fairly good swimmer when he quit formal lessons 2.5 yrs ago and is now a lot weaker. I'm not worried about the technical form of his strokes, he just looks like a weaker swimmer to me than I know he used to be (less command of his body, slower, not as agile in the water).
To me swimming's a fundamental, essential life skill, like knowing how to interact with dogs or when to say Please and Thank You. I'd see it as immensely derelict parenting if I didn't make sure DC were as competent as possible at those things.
And of course it's great fun, and I wouldn't want them to miss out on that.
We live on an island, ffs. I grew up in a surfer beach town, many outdoor swimming pools, no question about whether children "should" learn to swim.
It always shocks me (& scares the bejeesus out of me, to be honest) to meet adults who can't swim.