Is your seat right on the bottom of the kayak or elevated at all? You might also try filling a length of 3" diameter PVC filled with 20lbs sand or lead as ballast right on the center seam secured with epoxied straps. That way you can carry a light boat to the beach, the ballast on the second trip to get your paddle and pfd and still enjoy a skinny boat.

The solutions involving changing the hull shape get too close to trying to make a different hull that you might as well make a different hull. Sinking the kayak in the water with a little weight right on the bottom or lowering the seat can make a huge difference.