Also Sprach Ernie :
Is it normal for the battery gauge to read 16 when the boat is in
operation? The gauge only goes up to 18. The battery will be a year old
next month. Should I be concerned?

Regardless of the other replies, there are circumstances where charging
a charging voltage of 16-17 volts is perfectly normal and
expected. Specifically, many many older outboard engines, and even a few
newer but down-market ones, have low current unregulated charging
systems... in other words, they had a rectifier but no voltage
regulator. Since they are low current, topping out at 6 or 8 amps, they
typically won't fry your battery, so long as you make sure the water level
is topped up. Now, if you have an inboard or stern
drive, or a newer outboard with a regulated charging system, I'd do some
troubleshooting, starting by measuring the voltage at the battery with a
good digital voltmeter.

Dan

--
It's good to drink. It's good to laugh. But try to both at the same time
and you'll pay through the nose.