To make ahead: Cover and refrigerate juice for up to 4 days. Stir before serving.
Spicy Watermelon Juice: Cut 2 serrano peppers in half; muddle (seeds included) in a small pitcher until a chunky paste forms. Stir in watermelon juice; let stand for at least 30 minutes or up to 1 hour. Combine 2 Tbsp. kosher salt and 1 Tbsp. chili powder on a small plate. Run a lime wedge around rims of 4 glasses; dip the rims into the salt mixture, rolling them around to coat. Fill the glasses with ice. Pour the juice mixture through a fine-mesh strainer into the glasses. Discard solids. Garnish the glasses with lime wedges and sliced serrano.
Watermelon Lemonade: Combine 2 cups cold water, 1/2 cup sugar, and 1/2 cup fresh lemon juice in a large pitcher; stir until the sugar is dissolved. Add watermelon juice; stir to mix. Pour the mixture into 4 ice-filled glasses. Garnish with lemon wedges and fresh mint.
Watermelon Margarita: Combine 1 cup watermelon juice, 1/3 cup silver tequila and 1/4 cup fresh orange juice in a cocktail shaker. Add ice; cover and shake vigorously to mix. Place 2 Tbsp. kosher salt on a small plate. Run a lime wedge around rims of 2 glasses; dip the rims into the salt, rolling them around to coat. Fill the glasses with ice. Strain the juice mixture into the glasses. Garnish with lime wedges.