You'll want to start with the simple syrup. Add the water and sugar to a medium saucepan over medium heat. Stirring, until the sugar is completely dissolved. Set aside and let cool.
Take about 2 cups of fresh watermelon and run it through the food processor, you should end up with about 1 cup of watermelon juice.
Mix together the sugar and salt for the martini glass rim.
Rub some fresh watermelon around the rim, then dip the rim into the sugar/salt mixture.
Pour the watermelon juice, simple syrup, lime juice and vodka into a cocktail shaker. Add your ice and shake well.
Pour the watermelon vodka mixture into the martini glass and enjoy your watermelon martini!