The watermelon flavor is subtle, and the lime is a zingy, refreshing complement, making this the best picnic milkshake ever. Try to find the ripest, juiciest, and sweetest watermelon around. If you are using a watermelon with seeds, try to pick out most of them, but leaving a few is OK; they will get chopped up and lend a pretty, black speckle to the shake.
Excerpted from Malts & Milkshakes, by Autumn Martin (St. Martin's Press, 2013)
Remove the rind from the watermelon and cut the fruit into enough 1-inch pieces to measure 4 cups and enough 1/4-inch pieces to measure 2 cups.
Blend the 4 cups of watermelon with the sugar, salt, and lime juice until the watermelon is completely liquid. Add the ice cream and blend until it is just combined.
Stir in the 2 cups of smaller watermelon pieces.
Pour into glasses and garnish with a lime wedge.