'98 Smokercraft pontoon/'97 Mariner 2-stroke Bigfoot/Instrument problems

Bought the craft just before Christmas and the only instrument working is the tach, and it occasionally jumps around. Volt-meter shows voltage when key turned on, but goes to zero when started (wired backwards?). Nothing at all from temp gauge (most important to me). Fuel gauge shows half-full with full tank and starts going down when tank gets half down. Luckily I can see the fuel level in the translucent tank. Didn't notice any of this while in "new buyers" state of mind.

This craft appears to me now to have not been mechanically maintained, just cosmetically. Craft is in water at a marina, don't have a trailer. Any hints and tips to a "newbie" as to where to start attacking, especially the temp gauge? Just have the generic manuals for the boat and motor. Thanks.

Ayuh,.... My guess is ya got a Bad Ground, or Alota Bad grounds, or No grounds at all, at times....

Thank you. This gives me a starting point.