When I first started taking my daughter swimming, I couldn't work out why she was so tired afterwards. She would nap for hours after a "swim" and need an early night, but when she was actually in the pool, she did very little. All the other babies were kicking and splashing - my daughter would lie, motionless in the water, with a huge smile on her face and expect to be pulled around the swimming pool so she could watch the other swimmers. She looked quite regal in the process. She didn't move her legs, let alone kick - she was so busy watching everyone else swim, she didn't have time to do any herself!
It took me a while to work out that this was the reason for her exhaustion. She was having half an hour in a warm, noisy, echoey environment, surrounded by other people. She was experiencing new sensations in the water, and learning new language associations. That was what was tiring her out!
After many months of this, she began to kick and move herself through the water - and became even more worn out as a result! It was so interesting to observe how much an overwhelming new experience can tire our babies out though, and it's something I kept in mind when I had my second daughter and something I remind parents of during lessons. It's not the swimming itself that tires them out, it's the experience! So enjoy the time in the pool, take it slowly and let them observe what's going on around them - don't push them to do too much too soon. Both of my girls are good swimmers now, despite not taking to the whole physical side of the experience straight away.
And most of all, when you get home, put your feet up and enjoy the resulting nap!