[jmk]

As Capt George said, the batteries themselves cannot possibly get up to 16 V. Do I understand you correctly that when you use a hand-held multimeter to measure directly across the battery terminals, with the engines off, you see 16 V? And is the battery charger off? Is the ignition key off? Let's please sort that out first. Please forget about using your vessel's built in battery meter, as those are too prone to failure.

If all answers are yes, then the 16 V is coming from someplace other than the battery and this could potentially be a dangerous situation. Your next step would be to disconnect the positive (red) lead from the battery. Then confirm a direct reading on the battery terminals with a multimeter reads below 14 V DC as it must, and then measure both AC and DC volts between the disconnected positive battery lead and (1) the negative battery terminal and (2) the engine block. Both (1) and (2) should give identical results. Report any DC or AC voltage you get and that may give a clue to the problem.