Well I am having a similar but slightly different problem this time.

For the past 2 years we haven't been able to use the boat a ton due to having a baby however last season we took it out 3-4 times and everything worked great. Yesterday my 2 year old daughter kept saying she wanted to go for a boat ride so we got the boat ready and decided to be brave! I prepped the boat in the driveway and ran it on the fake a lake. I watched the motor come up to temp and for an instant the gauge needle seemed to climb slightly past 180 but then thermo opened and the temp dropped nicely, water was pouring out of the exhaust and prop shaft seal as expected, ran smooth, etc. Upon getting to the lake and launching I had probably made it 400-500 yards from the dock when I noticed the temp gauge was pegged totally hot. I immediately shut the boat off and did not notice any burning smell or smoke and nothing seemed excessively hot to the touch. I needed our boat ride and slowly drove back and trailered. This happened during the day with no use of nav lights.

After getting home I let the boat sit 2-3 hours in the driveway and even with turning the key the temp gauge was still pegged. I decided to run it again in the driveway with my IR thermometer shooting temps and everything seemed appropriate, the temp near temp sending unit never climbed over 155 and the boat ran smooth for 20 minutes or so. So this lead me to believe I am back to an electrical/gauge/temp sending unit issue.

In the past I had figured out the temp gauge would peg with turning on bow nav lights however now it remains pegged with bow nav lights on and off. Nothing I could do would get the gauge to function normally again. Since the temp gauge backlight bulb was the issue last time I checked this. The gauged were all initially backlit when turning nav lights on (including temp gauge) however with wiggling the temp gauge backlight housing then all gauge backlights went out (wired in series?) Still temp gauged remained completely pegged hot.

My daughter really did great, loved the boat and wants to go again! We don't really want to take her out again until the temp gauge is working reliably. So any and all advice is greatly appreciated as I'm lost on what to do next. Earlier in this thread someone had given advice that there was a potential grounding issue with the lighting and the temp gauge and suggested making a temporary jumper to bus bar with gauge positive and ground which I plan to try. It appears that replacement gauges are hard to find and may not be exactly plug and play. If you were me would you try and replace gauge, temp sending unit from motor?

Thanks for taking the time to read such a long post and any advice/tips/knowlege is really appreciated!