Freeze seedless watermelon chunks until completely solid.
Run a lime wedge around the rims of two margarita glasses and dip lightly into coarse salt.
Add the frozen watermelon, tequila, orange liqueur, lime juice, and agave syrup to a blender.
Blend until thick and smooth.
Add a small amount of ice and blend again if you prefer a thicker frozen consistency.
Taste and adjust the sweetness or lime juice as desired.
Pour immediately into the prepared glasses.
Garnish with fresh lime and small watermelon wedges and serve.