Watermelon is the perfect ingredient for a sweeter-style salsa. The two star ingredients in this salsa, though, are really the spicy Padrón pepper and the mint. The pepper adds a nice punch and the mint gives it complexity and pairs so well with sweet watermelon. Most Padrón peppers fall on the mild and sweet side but not all. Proceed with caution.
One thing I definitely recommend doing is straining the watermelon after chopping it to remove the excess juice.