Put the watermelon and juice from the limes in the blender. Blend until smooth.
If desired, strain the watermelon mixture through a fine mesh strainer using a spatula to push it through. This will remove any rogue seeds and chunks of watermelon. This is recommended if you want a very smooth drink texture.
Fill the glasses with ice.
Pour 2 ounces of vodka into each glass over the ice.
Pour the watermelon mixture into the glasses until 2/3 full.
Top off with ginger beer. Stir to combine.
You can use any ratio of watermelon to ginger beer to vodka that you like.