Pontoons

I just got into a debate with a pontoon boat owner who is having trouble keeping his pontoon completely afloat. It continues to sink about 1/4 to 1/2 of the way into the lake due to a slow leak. His plan is to drill a valve in the top of the float and add compressed air as necessary to force the water out of the pontoon. It seems to me that this is very similar to what we do with ballast tanks. Our argument stems around physics. He claims that since the pontoon boat is so heavy, he will have to blow more air into the pontoon to push the air out of the tube than if his boat was empty. His argument is that somehow, the heavier the boat, the more force is required to lift the pontoon, since the weight of the boat is pushing down on the float. To MY way of thinking, the pontoon will be at essentially 1atm ( as it is never more than 50% submerged ). Currently, the pressure of the water coming in must equal the pressure of the air in the float or it wold continue to sink. That being said, the amount of pressure needed to force water out has to be related to the size of the float and not the external pressure applied to the top of the tube. In my example, the amount of air/pressure wold be the same if the boat were completely empty or weighted 10 tons. In the ten ton scenario, the float might not have enough buoyant force to lift the boat, but the amt of air needed to completely clear the float of water would be the same as it would in the empty boat. I envision a case where the boat is SO heavy, that the pontoon float isn't even above the waterline when COMPLETELY filled with air, but again the amount needed to clear this tube would be the same as an equally filled tube in an empty boat.