Water Temperature Gauge Not Working

Look at the back of your cluster. There will be some screws going into the printed circuit board which ground it to the cluster, which is supposed to be grounded through the dash. Use some stake-on's and wire and make a ground lead. Attach one end to the screw I mentioned, and the other to the dash frame somewhere nearby so you can still get the cluster in and out easily. That's been the problem in all my cars, not being properly grounded.

Also, you seem to have it in your head, "why does the speedometer work?" It isn't electric, it doesn't have anything to do with the fuel gauge. It's mechanical, and totally separate.