Watermelons were originally native to Old Earth in Solsys. The original species, Citrullus lanatus, a member of the Curcurbitaceae family, evolved in the southern part of the continent of Africa. According to various surviving literary references the watermelon appears to have been already in cultivation in many parts of Old Earth long before the Information Age. A great annoyance to baseline humans in eating watermelons was the excessive numbers of large seeds. To rectify this problem, the Gyncloe Biotechnology Corp. gengineered a watermelon whose seeds are contained within the rind instead of the fruit. This simple gengineer, which occurred in 127 A.T., fixed the problems of the so-called 'seedless' watermelons of the 20th and 21st centuries c.e. (which were actually not seedless, but just had a small number of soft, sterile seeds which were easy to bite into and consume) that couldn't reproduce on their own.
Rind seed melons have no seeds whatsoever within the edible part of the melon. In the subsequent millennia since the creation of the first modified melons, they have been gengineered to grow on many different species of plant, and are available in many sizes, shapes and flavours.
While illegal in certain polities, certain species of watermelon that contain alcoholic fluids of various flavors and/or other hedonic drugs are very popular throughout much of Terragen Space. For this reason certain cultivars of watermelon are associated with hedonistic cultures.
The uka fruit, commonly found growing on Motherwood trees, is a heavily gengineered watermelon with high nutritional value. There are even provolved versions on some Pansophontist worlds, although it is not generally considered polite to eat them.