Why would you capsize a boat?

My Humans all went away shopping today, the forecast was so bad that as far as I was concerned it was a duvet day! When they came back it was decent enough to go for a walk.
The blip tonight worries me somewhat! The Back Up Human was at lifeboat training and they were capsizing boats, then swimming under them then righting them. I sometimes just don't understand Humans. Surely you go out in a boat to stay in it and keep dry. This was taken just off Craigleith Island. I just don't do water or boats, Luv Ginnie Xx.