Pack a large blender full of cubed ripe watermelon (with or without seeds) and blend until you have a juice. One small watermelon should yield roughly 3-5 cups (720-1200 ml), so you will have leftover juice (as original recipe is written) for extra margaritas. You can choose to strain the juice through a fine mesh strainer to remove any seeds or pulp, but I didn't find it necessary.
Line the rim of two serving glasses (amount as original recipe is written // adjust if altering batch size) with lime juice and sea salt (optional). Add ice and pour over margaritas.
*Nutrition information is a rough estimate calculated without salt.