St. Lucia has fantastic Rum Punch. It's hard to go wrong. Highly recommended:

• Chairman's Reserve (cask-aged dark rum)
• Crystal Lime (clear rum infused with lime)

Most bars will have both, even at the smaller resorts.

In addition to fantastic rum, Piton Lager beer is is brewed and bottled on the island and is very good (although it seems to have a higher alcohol content than most American beers...be warned!). If you're a beer drinker, you must have one. If you're not at a resort that offers it, check in coolers at the marketplaces - you'll likely find it in there for $1 US.