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.
To make margaritas, add watermelon juice, lime juice, tequila and a handful of ice to a cocktail shaker and shake vigorously to combine. Sample a small amount and adjust flavor as needed, adding more watermelon juice for sweetness, lime juice for acidity, or tequila for booziness.
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.
Best when fresh! Leftover watermelon juice will keep covered in the refrigerator up to 3 days.
*Nutrition information is a rough estimate calculated without salt.