This field is for growing watermelon. There are about two watermelon(s) per square meter. . We cut each watermelon into 18 slices of watermelon. Have a slice. Better yet have two slices (of watermelon). . What is this stuff, sliced up on the plate? It is watermelon. It is watermelon slices. It is slices of watermelon. It is slices of a watermelon I bought earlier. . Such a great spread. You seem to have thousands of slices of watermelon. Yes, many watermelon(s) suffered to provide this treat.

"I'd like that slice of watermelon, please."

A watermelon refers to the whole fruit. When you cut it up you have watermelon or slices of watermelon. If you point to a slice of it you can call it 'watermelon', but not 'a watermelon'.