I also taught swim lessons and coached swimming for many years through high school and college. I eventually was in charge of swim lessons for a large private pool. Every summer I would have kids (almost always boys) who were about 7-8 years old who couldn't swim show up for lessons. Now, this is AZ, so everyone has pool parties for their birthdays. These boys started getting invited to pool parties and now feel terribly ashamed that they can't swim. Always. Always. The mom would say "I tried it when he was 3 or 4 and he hated it so I took him out after 2 lessons."

Then they would complain that he was placed in a class with the 3 year olds, when that was his level. I would try to coordinate one class time for all the older boys, but then they would complain that the time was no good.
My son had crazy colic. The only time he didn't cry was when we were at swim lessons. It helped that my local swim school offers free lessons for babies under 6 months, that is how strongly they believe in baby swimming. I am sure it gets more babies in the 6-18 month classes too! But, he is almost 8 months now, has NO issues going under, can glide (with my help) to the wall and grab it and just this week started holding onto the wall well enough to "hang there." It is great!