Yep, watermelons are also berries... google it:
"A berry is " a simple fleshy fruit that usually has many seeds". Now there are three kinds of "true berries": (1) Smooth-skinned berries with entirely edible pericarps, such as
blueberries, currants, gooseberries, cranberries, grapes, tomatoes, and dates. (2) Round, leathery rinded berries, such as
oranges, lemons, limes, and grapefruits; they are the citrus fruits, referred to as hesperidia. (3) Elongated, many-seeded berries with a tough rind such as
bananas, cucumbers, watermelons, and other members of the gourd family; they are referred to botanically as pepos.
Now what about the seedless varieties. Are they berries? And where in the heck do the seeds for seedless watermelon come from? Back to your seats class for a quick biology lesson in genetics. First, yes they are still technically berries. Seedless watermelons are actually a hybrid form of watermelon called a triploid in which the seeds do not fully develop. If you look closely, you can see empty white "pips" where seeds normally can be found.
The seeds come from crossing normal diploid watermelon with tetraploid watermelon. Tetraploid watermelon have four sets of chromosomes, diploid watermelon have two sets. The offspring of this cross have three sets of chromosomes. These offspring have seeds, but the seeds produce plants which do not produce viable seeds."
http://www.hungrymonster.com/foodfacts/food_facts.cfm?phrase_vch=watermelons&fid=6064