Clive, you must be kidding me. Yes, I consider watermelons as a fruit.
I think I know why you asked that. Similar questions: Is honeydew a fruit? I think it's related to my following question
We say I ate strawberries, I ate blueberries, I ate bananas (if you really had more than one), I ate strawberries but I ate watermelon (singular)
Do you think the same principle hold for my original examples?
Watermelon[s ] is/are my favorite
Pineapple[s ] is/are my favorite.