!. No gas in tank? 2. sending unit "arm" or "bobber" damaged OR hung? 3. resister wraped sensor corroded or damaged? 4. wiring faulty (SO many ways for this to happen). 5. improperly wired at guage terminals? 6. poor or no current? 7. bad guage, busted needle?
You haven't said... 1. New to boat & have no idea if it ever worked. 2. was working untill some "event" took place. 3. Have borrowed a guage & things work. 4. Have aquired another sending unit & old guage works. Have a volt meter AND???? 5. Know how to read resistance in a wire. 6. Have removed old sending unit & inspected it.
Hope these things help, because I get no "feeling" by looking at the pics. (just kidding), the pics show a average installation & wiring. Hey! It don't work now right? So tear into it, you can't hurt it. Spray everthing with a penetrant overnight then check for "continuity of the wires. THEN the voltage into the tank. Then take the sender out and check for vusable problems. THEN, "ohm" the resestence at the sender. Then hook up a borrowed guage. THEN it's time to check the least likely, the guage itself.