1/2 oz. fresh lime juice
1 oz. sugar syrup*
1 1/2 oz. dark Jamaican rum (or more to taste)
2 oz. soda water
1. Pour ingredients into a tall glass packed with crushed ice; swizzle until well chilled.
* Sugar syrup: Heat equal parts white sugar and water until sugar dissolves, then cool and store in a bottle. Sugar syrup is sold at liquor stores and gourmet markets as "rock candy syrup" or "simple syrup."