Actual watermelons themselves aren't nasty, nor do they really smell gross.
It's that manufactured, watermelonesque smell from perfumes, watermelon-flavored bubblegum, and the like that have that cringe-inducing quality to it. Blame the "Flavor Corridor", which is the main stretch of I-95 through northern New Jersey. This is where about 90% of the world's "flavor" is researched and produced, including the "beef" flavor of McDonald's "hamburgers". It's also one of the main reasons it fucking smells when you hit Jersey.
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The giant three-phallused phallus of Uzbekistan will one day squirt the cosmic jizz of revenge all over Canada.