Have you checked the wires coming out of the sensor at the rear of the hull? See if you have resistance as you spin the small wheel. There should be 3 wires...I forget the color of the 2 to check, but 2 are for the spedo and the other one is for a temp reading on the dash if it has guage for that. If it reads no change as the wheel is spun it is in the sensor. If it does, move up to the guage and test the wires at the guage too. If all is good there too, then its the guage. Either the "Parts" link at the top of the page, or E-bay...or last resort, your local dealer.
Karl