The only consistent ingredients are the Goslings Black Seal Rum (defines it as a truely Bermudian drink), another light rum (Originally a Barbados rum), and orange and pineapple juices - with most opting for falernum over a basic sugar syrup. The rest is up to the drinkers personal favourites, personally I like Grenadine and cherry juice in mine to add some deeeper colour, and stronger flavours allowing for more Goslings to be added without noticing a change in strength.
Perhaps my favourite cocktail. First made at the Swizzle Inn the 1930s, but has been adapted by locals from the original recipe. Therefore across the island every family has their own mix with differing quatities or ingredients (for example the Swizzle Inns own recipe calls for triple sec, and some like to include cherry juice).