Firstly, follow the base game drinking rules, found here:

Additional rules:

• Any references to Longest Road, should now be read as Longest Trade Route instead
• Whenever you build your first settlement on an island other than the main island, any player who doesn’t already have a settlement on that island takes a shot
• Whenever the Pirate is placed on an Ocean Tile on which you currently have at least one ship or on an Ocean Tile adjacent to another Ocean Tile on which you currently have at least one ship, take 3 sips.
• Whenever you gain resources from a Gold Field Tile, give away a number of sips equal to double the amount of resources you received from that tile.
• Whenever you are about to move a ship, before moving it, count the number of ships in the trade route which that ship is part of. Then, take that many sips minus one.

Why it’s a good drinking game:

Yo ho ho and a bottle of rum! With the seafarers expansion to the Settlers of Catan, the expression “first come, first serve” takes on a very literal meaning, as you get to serve up shots to your opponents if they’re late to the exploration-party. On top of that, the gold fields and the pirate allow for new ways to both feel the rewards of settling the foreign shores and risking the punishment of a pirate raid on your trade routes.