Place watermelon cubes in a single layer on a sheet pan. Freeze at least 5 hours or overnight.
Zest limes; set aside for garnish. Juice limes until you have 2 tablespoons juice.
Place 1/2 of watermelon cubes in food processor with 1 tablespoon honey and 1 tablespoon lime juice. Blend until smooth and creamy and transfer to a large bowl. Repeat with remaining watermelon, honey, and lime juice.
Stir the 2 batches together and serve immediately, garnished with lime zest and Tajin. Or, place in a freezer-safe container and freeze for a firmer texture.