That’s a great question, Susan. The English language is definitely a tricky one, especially when it comes to the plural form of certain words. A goose by itself is a goose, but if you have more than one, you’ve got geese. But more than one moose is still called moose (not meese or mooses). Then there’s the mouse, which get together to form mice.
As far as I can tell, here are the rules regarding watermelon:
If you’re talking about wedges, slices or multiple pieces of watermelon, you should use “watermelon.” (EXAMPLE: “In the cooler, you’ll find hot dogs, hamburgers, and watermelon.”)
If you’re talking about multiple whole melons, you say “watermelons.” (EXAMPLE: ”I bought four watermelons for our barbecue, three of which are for me!”)
When you use a number, like in the multiple melons example above, adding the “s” is pretty natural. It gets kind of confusing when you omit the number and start talking about watermelon in a general sense. For instance, if I bought four watermelons for the barbecue, I might be able to say “I bought a lot of watermelon today. Probably more than I should have, but I’ll definitely eat the leftovers!”
In that example, I’m talking about watermelon in a general sense, which makes the singular form sound better, does it not? I hope this insight helps!