Low-poly Pirates from a first-person view. Possibly quite trite, but I'm not big on the genre in general. Haven't played Black Flag more than an hour.
While commandeering the ship you have to cycle through three types of ammunition: chains immobilize enemy vessels, buckshot halves personnel, and cannon balls damage the hull. Sounds familiar? The combat is addictive, but gets repetitive quickly. The best tactics is to always immobilize first, sink later.
If you get close enough to an immobilized ship, you can also board it, turning the game into a FPS version of tug-of-war. The best part of boarding is to be able to wield two pistols akimbo. Same mechanic works while capturing enemy ports. You run and shoot enemies until they run out of reinforcements, trying not to die.
Back at port you can replenish your lenient crew, fix your ship and pick one of the three bounties, which are always the same. Bounties are a great way to earn both money and experience. Also, you can capture ships that are way above your level, and all that for free.
You can have up to three additional ships in your fleet. The only commands you can give them, though, is to either sink or capture enemy vessels. No retreat. Which means you'll have to replenish your fleet often. With having an extra ship or two there comes another interesting mechanic - you can board enemy vessel from both sides, securing your victory.
Apart from fights, there isn't much to do. Trading is very rudimentary, with just three types of goods, and every port importing one type and exporting another. The story is short and doesn't yield even "congratulations" once you complete it.