2007 Tracker Tunda - 200 Mercury Optimax Voltage Issue

I recently swapped batteries on my boat and am now having issues with the voltage meter gauge stuck up around 15V even with the power disconnected. When I turn the power on, the volt meter goes past 18V all the way to the end of the gauge. The battery is charging fine but seems as if the alternator will not charge the battery when the boat is running. I checked all the fuses between the pos and neg lines and all are ok. I also checked the fuseable link in the motor and that is also fine. Any ideas on the problem? The volage meter gauge will not go below the 15V even with the power off. All pos and neg lines are connected properly to the battery.