Foam
Sea water has a density just under 65 pounds per cubic foot. You can probably get a foam weighing about 5 pounds per cubic foot which will give a net buoyancy of about 60 lbs/cu ft. For your 2200 lb boat almost 40 cubic feet of foam would be required. Denser foam will require a larger volume. This will only allow the boat to float with decks awash and the hull and cockpit full of water. In bad weather waves would be breaking right over the boat and the risk of being swept off the boat would be very high. This amount of flotation would be about 2' x 2' x 5' on each side of the boat. Spray on, conforming foam rather than rigid foam is probably your best bet. Don't worry about the water in the boat, it has the same density as the water outside and doesn't weigh anything when submerged. The alternative is an inflatable bag of about the same volume and a CO2 bottle to inflate it. Don't use your life raft because if it doesn't work you will have lost both the boat and the raft.
