Why not verify that the sender is bad before buying one!. Disconnect the sender wire (pink usually), turn on the key and then touch the pink wire to ground. The gauge should peg full if all else is ok. If the gauge doesn't peg, verify there is 12 volts on the "I" terminal of the gauge and that there is a good ground on the ground terminal. Jump the "S" terminal (sender) to ground and the gauge should peg full. If there is indeed 12 volts present and ground is good but the gauge does not peg, it's the gauge that's bad -- not the sender. To verify if the sender, remove it, using an ohmmeter, measure resistance from the center "S" terminal and the metal shell of the sender. As you move the arm up and down resistance should change from approximately 30 ohms to 240 ohms. If those numbers are way off or no reading you need a new sender. They are generic and are available right here on IBOATS -- the company that provides this free forum.
