If the boat is over dirt it won't take damage. If there is a rock (even a hidden one), stump, log or other object under the boat it will. My Karve has been parked for over 500 game days on the beach (I've been using a longboat instead) and still doesn't need repairs. I do the same thing with my long boat in the plains. Its parked on an open beach with no obstructions as I wander around and kill fulings. I put the black metal scrap in the ship and then portal back to my base. The boat will sit for 3 or 4 game weeks before I accumulate enough ore to make it worth sailing back to my main base. As long as there are no objects under the boat it takes no damage and does not need repair.