To make the fresh watermelon juice, puree 3 to 4 cups of watermelon chunks in a blender or food processor. Pour through a fine mesh sieve and press with a spoon to release all the juice.
Combine the sugar and 1 cup of watermelon juice in a saucepan over medium heat. Bring to a simmer, whisking so the sugar dissolves. Simmer for 1 to 2 minutes, then turn off the heat. Stir in the remaining watermelon juice, lime juice, tequila and grand marnier.
Fill each popsicle mold with a lime slice. Pour the watermelon mixture over top. Freeze according to your popsicle maker’s directions, for at least 4 to 6 hours. Serve with a sprinkle of sea salt.