When my daughter and then son started swimming lessons, obviously they couldn't swim but were still given certificates/badges (Ducklings, Puffins, etc.). These are swimming certificates for non-swimmers that have gained confidence in the water (i.e. can put their face in the water).
My daughter is 9 and has just swum her 900 metres and is in the swimming pre-club (where they train for the club).
My son who is 5 has just swum his 50 minutes (two lengths of pool, one on front, one on back) and did really well.
The club where they go swimming also do the National Swimming Awards (which start off at level 1 and progress to level 12 (I think)).
When my daughter (then aged 4) started swimming lessons at the local sports centre, she did fairly well but didn't progress as well as I thought she should. When my son turned 3 I put his name down for swimming lessons at the same sports centre, but there was a 1 years waiting list.
I eventually found a privately owned swimming club and I cannot recommend them highly enough.
When my daugther left the local sports centre swimming lessons, the girl who was teaching them had 13 children in her class (all aged rougly 6/7) so they didn't get much tuition in their half hour as the "young" girl had no control over the children who she was supposed to be teaching (for example, my daughter would be doing "twirls" in the swimming pool, instead of whatever she was supposed to be doing).
Since my daughter and son started at this new club, the tuition is brilliant. There are no more than 5 children per teacher and if a new child joins, they recruit a new teacher.
My son has half hour lesson once a week and my daughter has one and a half hours training once a week.
I think being able to swim is so important for all children.