Generally seedless watermelon is

Generally, seedless watermelon is Triploid. They have three sets of chromosomes. This odd number results in them being sterile and not producing seeds. The way they become triploid is by mating a diploid male with a tetraploid female.
The ordinary seeded watermelons are Diploid melons, meaning, that a typical watermelon has 22 chromosomes (diploid) while a seedless watermelon has 33 chromosomes (triploid).