We haven't felt swim lessons are a necessity, but they're nice when they work out, and the child is motivated and ready for the input. Fiona has never had any swimming instruction other than guidance from me and her siblings. She has been comfortable in deep water for a year and a half or so, and happy submerging for several years. But she's still mostly a dog-paddler, albeit one with a pretty cool self-styled backstroke. She's motivated to improve at everything, though, and swimming is no exception.
Our lake is only realistically swimmable for a couple of months a year, and we live a long, long way from a public pool. So it's nice to seize opportunities when they arise. A friend of ours is a swim instructor living in the next village over. She offers swim lessons in their lake (a little colder even than ours) every summer. This year the timing worked out, and lessons weren't full before we found out about them. It's a 75-minute drive, round-trip, but we go in the morning before the rest of the family gets going for the day. Fiona reads aloud to me from our readaloud book. We chat. We watch for bears, and moose, and coyotes, and great views, and raging creeks.
The first two days were SO cold! About 13ºC in both the air and the water, and windy to boot. Fortunately there were a couple of extra wetsuits available for loan, one of which fit Fiona. And a swim cap has made a huge difference.
She was very ready for this instruction! No fear, all eagerness, some decent confidence in the water. The stroke instruction is something she's internalizing really well. She's got the beginnings of a really good front crawl, and already looks like an expert in the back crawl. All in just three half-hour sessions.