Age of Booty

There are two ways you can unlock this achievement: against the AI or in a multiplayer match. The first method is not very difficult and can be done while going for the challenge achievements. The multiplayer method is very simple and shouldn't take more than 5 minutes if you want a really quick achievement.

Against the AI
I've found the best way to get this against the AI is to capture a town with only one access and attacking hex point and wait until the enemy attacks it. (There is such a town on the east side of Medium Challenge 4.)

First you'll have to get a Whirlpool curse by destroying a merchant ship. It's not very difficult, and, unless you're extremely unlucky, you should be able to find one within the first couple minutes of a game. If you find other curses before a Whirlpool curse, use them before trying to find a Whirlpool curse as you can't hold more than one curse at a time. Next, you'll need to capture the east town. It's a pretty weak town and close to your home base, so it shouldn't take very long.

Once you've captured the town, you need to wait for the AI to attack it. It's a good idea to keep your ship close so that the town is not captured while you're waiting for your ship to move next to the enemy. The AI is pretty quick to attack the town; I found that my town was attacked within 2 minutes of capturing it.

Once your town is attacked by the enemy, move next to the enemy to prevent them from moving off of the Whirlpool, then drop the Whirlpool on the hex they're occupying. As long as the town has high enough defenses to withstand the assault before the Whirlpool is dropped, the curse should be successful and the achievement will unlock.

Multiplayer Match
This is a very simple method, and possibly the easier of the two. You can unlock this achievement in a local multiplayer match with a second controller and account. Go into the Multiplayer menu and select Local, then join the game with the second account. (Make sure the accounts are on different teams. Otherwise this won't work.) Change the settings for Curses, disabling everything but Whirlpool. This will ensure you always get Whirlpool when destroying merchant ships.

Start the match and capture a town with your main profile. You can even use the secondary profile to attack the town as long as it doesn't capture. Once a town is captured, you'll need a Whirlpool curse. Destroy a merchant ship and pick up the Whirlpool crate. Then have the second account move next to your town and start attacking it. Once it's on that hex, drop your Whirlpool to transport the ship and the achievement will unlock.

Destroy a merchant ship and pick up the crate it drops, if its a whirlpool curse you're in luck. Wait until you spot an enemy attacking one of your own towns and then use the curse on them, they will be thrown across to another random place on the map. Achievement ahoy!