Get the pirate theme rolling right from the start by sending out fun, themed invitations. Buy little plastic bottles and send out "message-in-a-bottle invitations." Write out the invitations on parchment paper or print them out using a pirate font (yes, there's a pirate font). Roll the paper up and put it in the bottle -- along with a little sand or a tiny seashell. Seal the bottle with its cork (and a little tape to be doubly sure it's sealed). You'll need special postage for this invitation, but it can be done.
Or, how about drawing up a treasure map that leads to your house? Or, make paper treasure chest invitations that open up to show details about the party. You can slip little gold chocolate coins into each invitation as well.