Make the watermelon agua fresca:
Add the strawberries to a blender and puree until mostly smooth. Add water and puree until incorporated and smooth. Strain of its pulp into a sealable jar. Add sugar and give it a shake until the sugar is incorporated. Taste and adjust the sugar as needed.
Store agua fresca in the refrigerator until ready to use. This makes over half a gallon of agua fresca.
Make the cocktail:
To an 8oz glass add the lime, mint, and sliced strawberries. Crush with a cocktail muddler or a wooden spoon until the mint and mint is smashed - And juice of the lime slices are released. Fill the glass with ice. Pour agua fresca about ¾ of the way, then top with coconut rum.
Give it a little stir to incorporate. Sip.Sip.
Agua fresca makes about 8-10 cocktails.