Watermelon or watermelons?

...not in UK English.

Look at those watermelons.
Look at that watermelon.

A seedless watermelon is obviously inferior.
Seedless watermelons are obviously inferior.

I favor watermelons for the plural, but if someone used 'five watermelon' I wouldn't try to correct it.

..as in..?
Cum mister tallyman, tally mi banana ! (Harry Belafonte).

Depends on whether you're thinking of the material or the whole uncut watermelon.

A: I'm goin' to the store.

B: Good, get three watermelons for the party.

When a comparative reference to breasts is made, it pretty much has to be plural.

Well isn't watermelons the plural form of watermelon? So watermelon isn't plural right?

Good point, Minnie.