T.M.
I know you said you know about the Y, but I teach the class you are looking for at the Hatboro Y. It's called Perch+. It is not a "lets get used to the water class," it's a "your too advanced for the baby classes, but too young for the the preschool classes". I teach it the same way I teach the independent preschool classes, except the parents are helping. At my Y, it's an invitation only class, but 3 & 4 yr olds are usually just allowed to enter the class. As far as I know, our 2 Ys are supposed to have the same classes, so they should have it as well.
Good luck!
T.