Water balloons are Calvin's favorite summertime weapon, much like he favors snowballs during winter. Originally, he used his water gun, but it fell out of favor in the face of water balloons' superior power.
Water balloons are occasionally used in Calvinball.
Calvin often engages in water balloon fights with Hobbes. Although Calvin will put more effort behind soaking his adversary, he inevitably loses due to his habit of boasting. For instance, he once walked up to Hobbes with three water balloons, making much noise about his superior armament. Hobbes then dropped his own water balloon into Calvin's hoard, giving him no choice but to throw them all at once without aim.
When not battling at Hobbes' consent, Calvin still mounts offensives with his water balloons. These attacks are directed primarily at Susie and Hobbes, although Calvin has also soaked his parents on a few occasions.
Hobbes does not take these ambushes well, possibly given his fur coverage. He usually manages to dissuade Calvin from ambushing him, foretelling of punishment unequalled should Calvin dare attack him. Calvin once followed through, his curiosity piqued by Hobbes threatening retribution beyond the worst Calvin could imagine.
Calvin is far more inclined to soak Susie Derkins than Hobbes, due to her (official) status as his enemy. However, when he does so, he usually emerges as the loser since Susie violently attacks him in return. Susie has also learned to anticipate ambushes, as evidenced in a weekday strip where she donned a coat and umbrella before going outside.
In order to get Susie, Calvin will not only plan traditional ambushes, but also traps and mechanisms. These operate mostly out of his treehouse, and may or may not be a part of the G.R.O.S.S. agenda. At least one (a water balloon on a string) was a failure, and it is unknown how well the others worked. Calvin devises similar traps in winter, although these involve snowballs rather than water bombs.