I had this drink out at a bar and decided to recreate it myself. It rocks:
Take a cup of really ripe watermelon and put in in a blender or mortar and pestle and pulverise it. Then force it through a strainer to remove pulp.
Take 3 oz Watermelon juice
Juice of 1/4 of a lime
Sugar to taste
1.5 shots tequila
0.5 shots triple sec, cointreau, or Grand Marnier
Shake or Stir and ice it all down.
Makes a really amazing drink!
One further note. On the Cointreau debate. I just finished a bottle of Grand Marnier and found I really can't taste the dif in a Margarita. Likewise with a small bottle of cointreau I just finished. Bols make a very nice triple sec that does not have the artificial taste of some of the American brands for $7 for 750Ml which is a third the price of GM or Cointreau. I would save them for sipping over ice.