DD had swimming lessons from about 4-8 (and a spell of baby swimming). Swimming definitely has to be fun. DD's teacher seemed to come armed with inflatables and big floating mats for the last session of term so the children could just have fun. Intensive courses (so five sessions in a week) during holidays are also popular round here.
My biggest gripe would have been a combination of one or more of dirty/wet/cold changing rooms.
When the children were little (so feet couldn't touch the ground) I was also concerned about teacher to pupil ratios and having a lifeguard/helper/parents observing on side was a must for me (even if it is just one parent promising not to be engrossed in a book/phone as a rota).
I think a lot of schools have a system of "a badge/certificate per term" (three terms/year). If the children weren't ready to progress a stage they at least got a badge for distance in a particular stroke (even if it was just 10 metres), DD seemed to like that.
Another observation is/was that most children are pretty hopeless at translating instructions into technique when they are little. The number of times we watched children bobbing about in their own daydreams whilst the teacher thought they were explaining something (still happens at swim squad sometimes)! We asked the teacher to get DD out of the pool and move her arms/head/legs so that she understood what was required.
We had a variety of lessons starting with groups of 6-8 (but invariably a couple would be at parties/unwell), shrinking to 2 or 3 as well as some pricey private lessons where the instructor walked behind the pupil adjusting technique or ducked under water to watch a length/width. The latter were by far the most beneficial.
Good luck OP. It is such a useful life skill.