Many great suggestions, but I must protest the typically curmudgeonly suggestion –

From: ClintonHammond - PM
Gonna serve "Blow-Me-Down"? 1/3rd Rum... 1/3rd Gin... 1/3rd GUN POWDER!

If this is a subtle way of saying you need to spike the drinks with saltpeter (a principal ingredient of "pirate style" black powder), that's a really nasty trick. I think the army did that to me 40 years or so ago, … … …

… … and it's just now starting to work (erratically).

Of course you could use almost any appropriate "punch," preferably rum based for pirates, and salt it with something like the tiny licorice "cake sprinkles" and call it gunpowder, but you'd have to make it an "event" so that the "gunpowder is added and everyone gets some before the sprinkles dissolve. Proper label/disquise of your "powder keg" would help the effect, and you should avoid making all of your party punch taste nasty unless you try it out on a little to be sure it remains "drinkable."

(Old) John