Use a juicer or blender to extract about 3/4 cup of fresh watermelon juice from the cubed flesh. Pour the juice into a mixing bowl.
Roll the lime on the countertop to loosen the juice, then cut in half and squeeze to extract fresh lime juice. Zest a small amount of lime peel with a microplane, then add both to the watermelon juice.
Chop the cilantro into small pieces and add it to the bowl with the watermelon juice and lime.
Add honey (if using) and salt to the mixture. Stir well with a spoon or spatula until the ingredients are evenly combined and the salt is dissolved.
Let the salsa sit for 5 minutes at room temperature to allow flavors to meld. The salsa should be bright, slightly pulpy, and vibrant in color with visible cilantro leaves and bits of lime zest.
Serve immediately as a refreshing accompaniment to grilled chicken or seafood, or chill for 15-20 minutes for a cooler, more infused flavor.