When you have the boat on the trailer in your driveway, get a garden hose without the nozzle on and try wetting the hull, one area at a time. Have someone else be inside the boat and watch for any water coming in. This may take a while but you should be able to track down and crack big enough to let water in.
Did you check the water intake and any hoses you have running into the boat for ballasts, etc. Something may just need to be tightened up or sealed with water proff adhesive.