No, this is NOT normal under any circumstances. I'd suspect the meter first, i.e., you need to verify that the meter is reading correctly.
Second, you need to measure the voltage AT THE BATTERY TERMINALS. With the charger
off, and after a reasonable resting period, you should not measure more than 13.8 volts or so. Once you've bled off the "float charge", the voltage should be more like 12.7-12.8 with AGMs.
If somewhere on your boat -- any circuit -- you are in fact reading higher voltage than the voltage at the battery terminals with the battery charger off, then SOMETHING IS WRONG. Exception: if you are using a diode-based battery isolator AND if the charger is set up for it, you could read .5 to 1 volt higher or so than at the battery terminals (the diodes cause a voltage drop).
It's hard to imagine that a Xantrex charger would produce voltages as high as you are reading, even in the "equalization" mode (normally, about 15-16V). Solar panels
, unregulated, could do this.