A story arc from July 21st to July 26th of 1986 in which Calvin's mom signed him up for swimming lessons, which he vehemently protested.
Calvin didn't like having to get up so early for it (as the lessons started at 9am) and didn't like how cold the water was -- and unfortunately for Calvin, his instructor ended up being Rosalyn, his "sadistic" babysitter, who was teaching Calvin's swimming lessons as a way to earn money for college. One thing Rosalyn to invoke discipline, at least for Calvin, Rosalyn threatened to make use of a "rat tail" (a wrung wet towel used as a whip).
The first thing Rosalyn taught to Calvin and the other kids was the "Deadman's Float," which caused Calvin to freak out and start screaming for his mom. In the second-to-last strip of the arc, Calvin complains to Rosalyn about how he doesn't to learn how to swim, feeling that it's unnecessary. When Rosalyn questions what Calvin would do if he fell out of a boat, Calvin claims (and demonstrates) that he'll just wear a life-jacket.
In the last strip of the arc, Calvin's mom gives him a ride home and he spends the ride complaining to her about the swimming lessons, questioning her on why she didn't sign him up for something more exciting (like hang gliding or sharpshooting) or practical (like learning how to drive, even though he's legally too young for it). Calvin's mom then reveals that she signed him for piano lessons, which start the following Tuesday. This causes Calvin to start screaming out "No."