Bring water, sugar, and watermelon to a boil and remove from heat.
Use a potato masher to mash watermelon. Let mixture cool to room temperature.
Remove solids from syrup and store in an airtight container.
For the watermelon mojitos
Optional- Run a lime wedge around the top of the glasses and dip glasses in kosher salt for salted rims.
In two rocks glasses, divide mint leaves, watermelon, lime wedges, and 2 ounces of the simple syrup. Muddle together until mint is bruised and most of the juice has come out of the lime and watermelon.