Into a day of varied swimming activities comes the refrain of the title, encouraging Jim to join the others in the fun. ""SPLASH! Whoopee! Hey, look at me! I'm first one in, the water's fine! Sam don't howl. You've lost your towel? Never mind, I'll lend you mine."" The opening lines introduce an array of children, playing in a large indoor pool that has a wave machine and water toys. They show off their bikinis, whale imitations, and underwater antics. Poor Jim is reluctant to enter the water, a plot point that is obviously going to turn out well. There is plenty of humor, and the illustrations are downright jaunty, but the frenetically jolly text is wearing: ""Jim looks grumpy, gone all humpy. Says he doesn't want a dip. Going off to buy some biscuits--Mind, it's slippy! Jim don't trip!"" The endpapers show full-to-capacity children's dressing rooms in cheery colors, one of the nicest touches in the book.