Cut a lime into wedges, and use one to rub along the rim of two margarita glasses.
1 lime wedge
Roll the top of each glass in salt or sugar (whichever your preference is). Set aside.
1 teaspoon sugar or salt
In a blender, combine watermelon, tequila, sugar (if the watermelon isn't sweet enough for your taste), lime juice, orange juice and triple sec. Pulse until well blended.