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).

Thank you. What about pineapple and pineapples?

It's the same for anything that can be cut up from its whole form and has a counter like 'pieces' or 'slices' - pineapple, banana, pizza, cake, etc. 🙂

You know, we were taught that "cake" is uncountable.

Cake is only uncountable when it's cut up. Then you'd say pieces of cake. But whole cakes are countable - "the baker made three cakes for the party."

Balthasar, here is some more food for thought.

Fruit or fruits ?

If you had pineapples, watermelons, bananas, oranges and strawberries at home, what would you say?

1. I have a lot of fruit at home. Right or wrong?
2. I have a lot of fruits at home. Right or wrong?
3. I have many fruit at home. Right or wrong?
4. I have many fruits at home. Right or wrong?
5. I have fruit at home. Right or wrong?
6. I have fruits at home. Right or wrong?

When you finish this think about the word "fish" too in the same context.

Cheers!

Oh , what a lovely quizzes :-)