Blend: Blend watermelon until smooth, optionally blending in nutrition or flavor boosters (like fish oil, mussels, or sardines).
Freeze: If using silicone paw-shaped molds, set the molds on a baking sheet first. Pour into mixture into molds, leaving a little space at the top for them to expand while freezing.
Serve: Pop watermelon dog treats out of the molds and store in an airtight container in the freezer until ready to serve.