Tach issues point towards an electrical issue which you definitely have if you are getting that much voltage at idle. I would look to confirm the major components of your engine are working as designed starting with the stator and rectifier. It may be your gauges but unlikely. I had an issue with my stator that was causing low voltage and my tach to act strange with erroneous readings . Anyway, others will be by to lend their expertise but I am guessing that it will likely be centered around one or both of those components. As a side note: I would be careful if you are truly getting 16v because it could damage other components on your boat that are designed for 12v systems. Certainly not an expert here but I would be prudent until you know exactly how much voltage is being sent to the house system... don't want to add to the list of repairs you may eventually have.
B