L.J.
My three year old just finished gym & swim class at the willoughby Y, I really do understand your concerns, and I sugesst you attend the orientation for parents prior to the class ( its free and offered every session).. you will meet the teachers and get your questions asked.
The ratios were always good- two kids to one instructor and my daughter loved it, they seem to run a tight ship over there and my daughter has enjoyed several classes.