Watermelon breeders discovered that crossing a diploid plant (bearing the standard two sets of chromosomes) with a tetraploid plant (having four sets of chromosomes) results in a fruit that produces a triploid seed. (Yes, it has three sets of chromosomes.) This triploid seed is the seed that produces seedless watermelons!
https://www.watermelon.org/the-slice/where-does-seedless-watermelon-come-from/