Swimming, swimming / make a swimming motion
In a swimming hole / Make a hole with your arms
When days are hot / fan your face
When days are cold / hug yourself and shiver
In a swimming hole / Make a hole
Breaststroke / mimic breaststroke
Sidestroke / mimic sidestroke
Fancy diving too / pretend to dive
Now don’t you wish you didn’t have / shake pointer finger up and down
anything else to do / wave hands horizontally in front like NO
BUT!

repeat, each time leave off one more line and just do the motions.  By the end, the only thing said will be the word BUT