"I love watermelon." This is the natural way to say it.

Interesting question. As Peachey says watermelon is correct. I think because you would not usually eat a whole one yourself. Similarly for pineapple, durian, dragon fruit , pomelo and maybe grapefruit. I like peaches, oranges, apples, bananas and strawberries. As a countable noun - I need to buy two watermelons.