Run a slice of lime around the rim of a margarita or rocks glass. Dip the rim into margarita salt to coat. Carefully fill with ice, if desired, and set aside.
Fill a cocktail shaker with ice. Pour in the tequila, coconut water, agave syrup, and lime juice. Cover and shake vigorously.
Strain the margarita into the prepared glass. Garnish with a lime slice and a pineapple chunk on the rim. Serve immediately.
Notes
Use actual coconut water for this margarita. You don't want carbonated water with coconut/pineapple flavoring. (It'll make the cocktail shaker fly off when you shake everything together.) If needed, you can substitute pineapple-flavored tequila if you can't find coconut water with pineapple.