Cut the watermelon flesh from the rind into cubes.
In a blender, process half the watermelon pieces with 1/2 cup of water until smooth.
Pour through a strainer into a pitcher, using a rubber spatula or wooden spoon to press the liquid through the pulp and seeds. Repeat this process with the remaining melon and water.
4. Stir in the lime juice and optional agave or honey. Muddle some mint into the bottom of a glass along with some ice, and pour the liquid into the glass. Garnish with lime slices and mint.