We first took Andrew to swimming lessons when he was 6 months old. At that time, the main objective was to get him comfortable with being in the water. We held him, got him to kick his feet a little, and even quickly dunked his head under the water. He hated being dunked, and the water was a bit chilly so he didn't last very long.
Yesterday, we took him to his first "real" swimming lessons. This time, mom and dad were not in the water with him - just the instructors and about ten kids ages 3-5. He practiced floating on his back (with some help), blowing bubbles, and kicking his feet while laying on a floating pad.