Watermelons are destructible objects that appear several times throughout TimeSplitters 2 and TimeSplitters Future Perfect.
Challenge[]
They appear as the target in a couple of Challenges, specifically in Monkey Mayhem and Melon Heist. In said Challenges, you must shoot the melons out of the hands of monkeys, while not shooting the monkeys themselves.
Story[]
Watermelons appear in Siberia, though only as still objects, in the same sense as crates and drums.
In Aztec Ruins, soon after the level begins, you can find enemy monkeys throwing high-tech, explosive watermelons at you from trees, dealing a fair amount of damage. After a watermelon hits the ground, it will explode, just as if it was a grenade.
Watermelons can also be useful; if you can get a monkey to throw one at you during the face-off with the Stone Golems inside the temple, the exploding melons can kill the golem instead of you, without having to try tricking them to get into the trap doors on the floor.