| | Re: watermelon or watermelons?
If you have many pieces of cut-up watermelon, you could say "I am eating watermelon."/"I have eaten 4 pieces of watermelon."
If you have several whole watermelons, then "watermelons" is the correct plural.
Watermelon can be both countable (watermelons) and uncountable with a counter word (4 pieces of watermelon).