Fill a mixing glass or tumbler three-fourths full with ice. Add rum, guava nectar, lemon juice, bitters and simple syrup; stir until condensation forms on outside of glass. Strain into a chilled cocktail glass. Add ice and top with coconut water. Garnish as desired.
For nonalcoholic version: Increase coconut water and guava nectar to 3 ounces each. Eliminate bitters and add 1/8 teaspoon coconut extract.