Seedless watermelons are cross breeds better know as hybrids. Just like the offspring of a male donkey and a female horse, the mule is almost always sterile.
Mule Reference The
seeds for seedless watermelons are bred in a similar fashion. The pollen from one particular variety is used to fertilize the stamen of a different variety. The resulting
seed will produce fruit, but will be sterile.
A more scientific explanation is provided by wikipedia "Oddly enough, seedless watermelons are grown from
seeds. These
seeds are produced by crossing diploid and tetraploid lines of watermelon, with the resulting
seeds producing sterile triploid
plants. Fruit development is triggered by pollination and these
plants must be grown alongside a diploid strain to provide pollen."