In my (limited) experience, it's _low_ humidity that causes wooden
constructions to crack, or at least open up and leak.  High humidity
should not crack wood, but may cause glue joints to loosen and let go.
Cycling of humidity, between extreme moisture and dryness, after a few
seasons may cause glue joints to fall apart.

Here in Maine I've discovered that my two foot-pumped reed organs
(a Mason & Hamlin melodeon and an Estey WW2 chaplain's organ) can hardly
be played in the winter, but are easy to pump and play in the summer,
when the cracks close up.

Given that most carousels are open to the weather, rainy days probably
lay more moisture on the organs than a seaside or lakeside location.
Then there's the huge (manual) organ at Lake Chautaqua, NY, which plays
into an open-air amphitheatre.  I have no idea what holds it together
through the rainy seasons!

Mike Knudsen