i did take my 6 month old son swimming in public pools during public hours..
because the "big girl" was having her "big girl" swimming lession at that time...
if you do a seach on "babyschwimmen" and your area, you will find lots. They are an opportunity to meet people with kids the same age.

http://kidscorner.ch/kurse/schwimmen.html

If it really does make a difference to their development, i dont know.. but its fun