If so, your watermelon did not get pollinated well enough when it was only a blossom. The answer is bees.
Bees gather pollen and nectar that they find in the blossoms.
If bees don't go into the blossoms to forage for food, the blossoms won't make seeds. Many tiny yellow grains of pollen must land all over the sticky pistil of each flower to make the seeds grow inside the watermelon. You might think seeds inside a watermelon aren't important. Maybe you think they get in your way when you eat a piece of melon. But if you want a tasty melon, you want pollinated seeds inside.