It's not uncommon for boats to have connection resistance. How old is your
boat? Corrosion is the common cause. The dash voltmeter may be correct,
you might only be seeing 11.5 volts at the dash. Try your multimeter on the
terminals of the dash voltmeter and see what it says.

The boat is from 1995, around 10 years old.

Thanks for pointing out that the connection between the battery to the
dashboard voltmeter may have too much resistance. I will check the
voltage at the terminals of the dashboard voltmeter to see if it is
reading the same low voltage as the voltmeter. If this is the case,
this means I definitely need to do something about the wiring in this
boat that I intend to do anyway when the weather becomes warmer.

Jay Chan