I could never manage after-school swimming lessons - dc3 was 'challenging', and waiting with him while the others had their lessons, plus dealing with thechanging room business, was too much for me to cope with. Instead I would book the elder two into intensive courses, 5 lessons in one week, a couple of times a year during the holidays. They would advance hugely during the courses, and because we often went to the pool as a family, they could then practice what they had learned. Ds1 learned to swim by about age 6 this way, but when our family swim habit lapsed dd had not got yet the hang of it. At 6 she could do the mermaid-swim thing, handstands, tumble-turns, etc, but could not come up for air without stopping.
So this year all 3 are having eekly swimming lessons. We found a lovely swimschool that runs several different level classes at the same time, so that siblings can all have their lessons at the same time. Within half a term dd was swimming competently and was moved up a level. Both the elder dc are now strong, confident swimmers. Not particularly fast, but with good technique. 4yo ds2 still has some way to go. After one term he is where dd was at 6.
My mother was terrified of swimming, so she made sure we could all swim. She herself learned to swim in her 60s! And nearly gave our dad a heart-attack by jumping in at the deep end without telling him she could now swim 