The Caribeño is often attributed to legendary tiki barman Martin Cate, of Smuggler’s Cove fame – but his book Smuggler’s Cove: Exotic Cocktails, Rum, and the Cult of Tiki seems to indicate that it’s an older Caribbean recipe. Apparently, rum and gin are both traditional choices, although we much prefer the rum version.
The book also calls for Smuggler’s Cove’s own house Demerara syrup, a 2:1 syrup made with 3:1 white:brown sugar. You could use either type of sugar syrup, or your own mix – and even 1:1 syrup works perfectly well at the given amount, since coconut water has a bit of its own sweetness.