I have pretty strong feelings on this. You can never learn to swim too well and it's a skill that can be used and passed on for a lifetime. It's one of the most important childhood milestones IMO.
My mom had a pool put in when dd was 2, ds a baby. Dh and I started them swimming by hold nose, 1, 2, 3, wee, quick dunk under,with kick boards, later with torpedo diving. It was enough for them to learn a comfortable above water wade, doggie paddle. I never used arm floats.
My kids started swim lessons when they were 5. So well worth it. DD and I were in a near drowning incident river tubing 3 years ago and it was scary. As soon as we got back to the hotel, I took the kids swimming to not show fear. I really thought I was a decent swimmer, but my mind was changed quickly. I moved both kids into the advanced swim class which was 2 hours per week. Next dd did swim team, which is excellent for endurance and technique-6 hours per week plus meets. Last year we did a season pass at the local waterpark, 4 full days per week all summer plus they swim every warm weekend at my mama's. My ds can swim underwater like a fish and my dd built up stronger calves than a mule from her flip kicks. Well worth the $ for the confidence in the water.