Terry wrote:

The specified/allowed 'Voltage variation' plus/minus varies (pun
intended!) a little from country to country and region to region.
E.g. North America, Europe, etc.
In some places it may be plus and minus ten (10) per cent. In
another area it may be, say, minus 6% to plus 10%. If in Mexico
the voltage is occasionally up to 128/130 on a supply that is
supposed to be 117/120 volts; that's around 9% to 11% high. i.e.
acceptable? If you are monitoring it anyway why worry? Is in
danger of burning out. i so tap it down to the next lower
ampere/voltage output setting?
Or am I missing something?

Terry writes further.
Transformers tend to be heavy (if this is of any concern on an
average boat?). A 'lighter' way than an auto transformer or
tapped transformer which carries the whole load (which is
apparently of the the order 40 amps x 12 volts = approx. 480
watts; that's about 500 VA) is to use a smaller transformer with
a low voltage output.
For example lets assume a suitable transformer of 120 volts input
with ten volts output. That small transformer is connected in
such a way that it's output 'opposes' the too high voltage coming
from the shore supply; viz. 128 volts minus the approx. ten volts
of the smaller transformer is; 128 - 10 = 118 volts (almost spot
on!).
You have to know what you are doing but it can and has been done.
The small transformer in the example given will only have handle
about 10/120 of the total power = less than one tenth of the
total wattage and can thus be more compact, cheaper and lighter.
But it must be done safely and connected the proper way with
appropriate fusing.
Cheers.