First kill the guards. In fact, you can kill them as soon as you get out the door and they won't react at all. Now head on outside taking great care to kill the guys with chainsaws before they reach you. Clear a path to the boat in the dock. You don't need to kill them all, you only need to get to the boat and give chase. In fact, you can run past them all if you're fast enough without taking too much damage if you so wish. Now just destroy the boat. You can use the boat supplied with your own weaponry, or you could call your own boat, like the Cigarette Gun Boat for instance. Either way it shouldn't be too hard. Now go back to the Lounge and talk to the manager.
You can now buy the place for $8,000,000. If you have Montana Holdings it will cost you $7,200,000.