Acoustic guitars love humidity, preferably 40%+, but it needs to be relatively constant. This is especially true if the top is solid, not laminated. It’s low humidity, like we have in AZ, or sudden changes that cause harm.
I bolded two unfounded myths.
Here are more scientifically accurate statements:
The RH does not need to be relatively constant and sudden RH changes do not harm guitars. Wood used for guitar making has been dried before being formed into a guitar. Properly dried wood does not absorb or release moisture rapidly. Guitars can easily withstand diurnal RH cycles from 50% to 99%. In the winter, they can easily withstand transfer out of a room or closet or case humidified to 60% into a living room that is 20% RH for lengthy playing sessions.
Acoustic guitars do love relative humidity that is close to the RH in which they were built. They can be damaged by long exposures to RH significantly lower
or higher than the RH in the place they were made.
So, if you want to keep a wood guitar on a boat, check the monthly average humidity for the place that the boat will be and the months that it will be there.
The statements above pertain to boats that are not heated or refrigerated. For guitars kept in boats or buildings that are heated/and or cooled, the outdoor RH is not directly relevant.
If you raise the temperature of air, you reduce the RH. Therefore, heated spaces have lower RH than outdoors.
Refrigerated air has often had moisture extracted, but still leaves the A/C unit at near 100% RH. The chilled air mixes with the warmer room air and the resulting mixture can have a lower RH than before refrigeration. However, if your A/C unit is oversized, or you run the room temperature too low, the RH can get quite high in a refrigerated room.