Watermelon is a food item in the Kirby series, and Kirby's favorite food in the anime series, Kirby: Right Back At Ya!. They are often used to lure Kirby into King Dedede's or NME's traps - such as into the NME Transporter which transports Kirby to Nightmare Enterprises, or Dedede manipulating Kirby into a karaoke contest by making first prize a lifetime supply of watermelons. It was the first thing Kirby ate in the first episode of the anime, and he eats a lot of them throughout the entire series.
Watermelons are possibly Dream Land's prize crop in the anime, as cultivated watermelons are a common sight.
In the games, watermelons are one of the standard delicacies that restore Kirby's health.