"SonomaProducts.com" writes:
Dam fine question. Not really sure on that one.

My speculation: on flooring the underside is pretty concealed from the
environment, not getting any air flow for exchange of moisture in\out.
Also, I seem to recall one fo the forst steps in doing floors is to
determine the moisture content of the substrat and to apply moisture
barrier in most cases. In fact, certain floors are not supported for
us over concrete floors IIRC.

Well at least in the old days when the subfloor was just loosely spaced
and knotty wide planks the underside would be pretty exposed. On the
first floor, the undeside would get pretty good circulation with damp
basement air...

Boats, I guess they are going to get pretty saturated from the
ambient in a fairly consistent manner and kind of always stay at a
high saturation.

Though in the Northern climates, Boats get taken out of the water in the
winter so I would think the range in extremes from total saturation in
the summer to total dryness in the cold winter air in the winter would
be a lot more extreme than say a bathroom where the moisture level is
high but all-in-all pretty uniform over the time scale that wood
exchanges moisture with the environment.

Also, I would think that boats would be particularly susceptible to such
warping & stresses since the wood is often "bent" and applied under
stress. Plus the fitting needs to be particularly tight relative to
countertops and flooring where a little expansion/contraction/warping
may be ok.

n ... all speculation, but really just slighly more so than most of my
posts ;^)

All good and helpful speculation, but for me at least the question
remains...