It is a countable noun and can also be used as an uncountable noun. When it is used as a countable noun, it is plural.

Watermelon, meaning watermelon, is a compound word. Watermelon tastes juicy and can be counted as watermelon. Watermelon. Tip: As the name implies, it is a kind of melon from the western regions, called watermelon.

I bought a watermelon and some peaches.