In a blender puree watermelon. Press puree through a sieve or cheesecloth to free juice. Wet the rim of a chilled martini glass with a piece of watermelon. Dip the rim into a sugar and salt mixture. Place the watermelon juice, vodka, lime juice, and simple syrup into a cocktail shaker. Top with ice. Shake well. Pour contents through strainer into martini glasses. Garnish with a wedge of watermelon if desired.