Place the cubed watermelon into a blender and blend until completely smooth, about 30 seconds. Check for a uniform, bright pink liquid with no large fruit chunks.
Add honey or agave syrup to the blender if desired, then blend for another 10 seconds until fully incorporated.
Pour the watermelon puree through a fine mesh sieve into a pouring jug or bowl to strain out pulp and seeds, resulting in a smooth liquid. Gently tap or swirl to encourage liquid flow.
Divide the strained liquid evenly into silicone popsicle molds, leaving about 0.5 inch at the top for expansion. Insert sticks if not already attached.
Place the molds in the freezer and freeze for at least 4 hours, or until completely solid. The liquid should be firm and frosty when ready.
To serve, briefly run the outside of the molds under warm water to loosen, then remove the popsicles. Serve immediately for a refreshing, icy treat with a translucent, vibrant appearance.
