Woodwind instruments simply needs bit more care be it wooden recorder or harmonica.

The smallest part of the wooden cover plates is on the front where they meet the lips. The cover plate in this section must form an effective space for the vibrating reeds, while its outer shape remains shallow for ease of use when playing with the harmonica fully in the mouth. I say this openly because it will come to your mind when you see the covers for the first time. No worries, once you screw the cover to the harmonica body the whole thing comes together and becomes durable unit.
The back of all the wooden covers are reinforced with either “teeth” that bite back into the body, or the “Mouth” covers for the diatonic harmonica have a thick rib that gives it solid support and, in the case of the “Mouth” model enhances its beautiful sustain.
The back along the center of the covers has additional structural reinforcements that allow the harmonica to be held firmly while not dampening the vibrations you want to feel when playing.

Water and humidity
Again, nothing to worry about.
Yes, it’s not advisable to bath the harmonica in water for extended periods of time, but you don’t have to worry if it gets wet either. Boats are made of wood, airplane propellers were made of wood, car frames were made of wood, harmonica is no exception.
The cover plates have an adequate “food safe” finish that is resistant to moisture. There are two types of finish that I use. Oil and acrylic, more information follows.

Acrylic varnish is the least maintenance intensive.

From the moment you get the harmonica in your hands you don’t have to worry about anything. Acrylic lacquer is harder and smoother to the touch than oil varnish.
The disadvantages of acrylic varnish over oil varnish are the dampening of the wood’s coloration. In addition, acrylic lacquer can worn out on some parts after years of use and it is then somewhat tedious to repair these areas nice and smooth. After the years of use.

Oil varnish gives the wood an irreplaceable look.

The advantage of oil varnish is that it is applied in depth, the wood is better protected against moisture. By applying natural oil from time to time, its lifetime is extended almost indefinitely. Woodwind instruments simply needs more care be it wooden recorder or harmonica.
Personally, I recommend getting a lip balm, which costs next to nothing. From time to time the body of the accordion is very lightly rubbed with the balm and polished with a cloth or just by hand. Lip balms are made from natural oils or close substitutes and so are perfect for maintaining the oiled surfaces of the harmonica.
Small tip; The lip balm is great lubricant for the slider in chromatic harmonica.