For six drinks:
9 ounces white rum
4.5 ounces fresh lime juice
4.5 ounces mint simple syrup
16-oz bottle sparkling water
6 mint sprigs
For mint simple syrup (makes approx. 1 cup):
Small bunch fresh mint
1/2 cup white sugar
Heat up 4 ounces of water until boiling. Pour over cleaned fresh mint, and allow to steep for 15 minutes. Stir in the sugar, until dissolved. Strain out mint, reserving syrup, and chill until ready to use.
Indoors: Combine rum, lime, and mint simple syrup in a sealable container and stir to combine. Refrigerate until ready to go. If possible, keep container and sparkling water chilled in a cooler, with ice or chill-packs, at the beach.
Outdoors: Give the container a good shake, then pour out into six large cups with ice. Top with about two ounces of sparkling water. Add some fresh mint to each.