Jumping in...so to speak
I will no doubt incur the wrath of some...but this is, in my opinion, nuts, folks. Read the Tipping Point. Maybe it was Freakanomics. Swimming pools are 100 x more deadly than handguns to kids. [and I hate handguns] 85% percent of all drownings occur when adults ARE watching the kids...
Most lifejackets do not inherently keep the head above water. Only the ones with the added portion, behind. Life is going to be pretty miserable for the parent, and there are plenty out there who have experienced this, who has to watch their kid panic, or god forbid, go under, when things go awry...from the dock. And who among us hasn't had a sail when things didn't go as planned? I remember racing as a kid on a laser, two people, and we buried the nose doing about ten knots....flew about 30 feet.
Glad I was comfortable in the water.
I love the water. I love sailing. I love diving. And I wouldn't consider teaching having my kid out there by himself unless he was very comfortable in the water, could paddle and/or swim. As a matter of fact, my kid's in 8 weeks of swim lessons as we speak, because we think it's a priority. He's going with his best friend, daily, and generally having a ball.
Forcing a kid to do a talent show is one thing. Forcing a kid to play piano for years is another. But swimming lessons...I wouldn't even give that pause.
This is a lifelong skill that, statistically speaking, is probably as important as wearing a bike helmet. More than that, it's a skill that ties in naturally with sailing. I spent 4 years sailing through the South Pacific. How sad it would have been if I couldn't have snorkled through those perfect 84° waters. An entire aspect of the trip, I would have lost.
Parent in the boat, teaching? Fine. Otherwise, my suggestion, emphatically, would be that your in-laws should spend the money on swim lessons, poste haste.