Watermelon Ginger Cooler

Directions

1. 1
   Toss the watermelon into a blender or food processor, and blitz for a minute or two. After that you, you can run the juice through a sieve (which will ensure you have a smooth watermelon juice), or leave it a little chunky.
2. 2
   Juice one of the key limes into the watermelon juice. Give it a little stir. Slice the other, thinly and crosswise, and set it aside.
3. 3
   Empty some ice into a cocktail shaker. Pour the ginger syrup and the rum directly over the ice. Add the watermelon-key lime juice. Attach the cap, and give the cocktail a brisk shake, until the metal feels quite cold to the touch.
4. 4
   Pour the drink out into a glass, making sure to leave the ice behind. Garnish with the remaining key lime, and enjoy cold.