I still think your dash cluster may be the problem, you might try calling this company and ask them about your problem, they actually repair these panels:
http://www.cdielectronics.com/dash-panel/As I said earlier, hooking up a temp regular gauge to your present sending unit, with a +12 volts and ground hook up to the gauge will tell you for sure if its the dash panel.
If I had one of these boats I'd for sure get rid of this bad idea and re-do the dash panel with regular gauges that can be easily replaced.