I assume you managed to pull your boat over Thanksgiving? You probably have 1000lbs or so of pulling force with your jeep depending on traction. A 12,000lb winch will certainly pull your boat out. You only get 12,000lbs on the first turn on the drum and every turn after that lowers the pulling force, however you will have more than enough. A snatch block not only doubles the pulling force it uses more line so fewer wraps on the drum so you get more than double with a snatch block versus a single line pull.
The only problem I can see you running into is not enough battery power for the winch. If the jeep is on an island you probably don't run it much. Charge up the battery fully and be prepared to jumper the jeep battery with the boat battery (which you will take out of the boat for winter storage).