This combo is amusing thematically. You deploy Ambassadors from your Kingdom to other Kingdoms, gifting them Coppers*, which you then promptly loot with your Pirate Ships.
This combo hinges on constantly Ambassadoring your opponents with Coppers. Later on, you can start passing them Estates, but the key is to flood their deck early on with targets for your Pirate Ships. It has the nice secondary benefit of eliminating Coppers from your own deck, thus making it more difficult for their Pirate Ships to strike back.
It does dramatically slim your deck, which means that any Treasure you do have is more susceptible to attack. But as the Sample Game demonstrates, that’s not really a concern if you have the patience to work your Pirate Ships up to $8.
Masquerade works on the same principle. Mountebank is close; it’s obviously more difficult to acquire, doesn’t always trigger, and doesn’t purge your own deck of Coppers. But it doesn’t depend on drawing it with Copper, either, and provides money as well, always critical in Pirate Ship games.
*Technically, as Donald explains in The Secret History of the Dominion Cards, they are “Copper Mines”, but the name was shortened to “Coppers” for simplicity.
The Great Halls are a nice $3 pickup, since too many Ambassadors would interfere with the launching of your Pirate Ships.