DD's first swim was at Garnet at 4 months in Parent & Tot 1. It was great - the ratio is meant to be 1:10, but there were only two people signed up so there was much more personal attention :) We chose Garnet since that pool is one of the warmer ones.
DD's current swim classes are technically the "Preschool 1" level with ratio of 1:4 teacher:child. The class descriptions/ levels are in the
Recreation Guide. I'm not too worried about her safety - she's very familiar with the pool since this is where we take her for Parent & Tot swim on the weekends, and she's been in the water enough to know her limits. It's in the small shallow pool at North Thornhill CC, and when they do venture to the deeper side, the teachers put the PFDs on the kids. I am actually amazed at how well behaved and cooperative all the kids are during these lessons! There's usually 5 teachers/ groups with 4-5 kids per teacher, plus the Deck Supervisor/ Lifeguard.
DD's classes are first thing in the morning (once a week), so I usually stay and watch from outside the windows. I leave when the class ends and the daycare teacher showers the kids (to wash off the chlorine) and gets them dressed and back in daycare for the rest of the day.
Also, there's a previous thread on swimming here, though with much of the same information as this one -
http://www.lifewithababy.com/Anything_&_Everything_Else?mode=MessageList&eid=385318&tpg=4