From jasonobryan of drinksanddrinking.com, a really nice recipe and an even nicer explanation of why it's so good.
2oz Flor de Cana silver rum
0.75oz Demerara or muscovado simple syrup (1:1)
0.75oz fresh lime juice
2.5oz mineral water (salty and highly carbonated)
6-10 mint leaves
Add all ingredients except mineral water, including mint, to a cocktail shaker and fill three-quarters with ice. Shake vigorously for 10-12 seconds. Fine strain over crushed ice into a Collins glass. Garnish with a mint sprig.