Though not actually boardwalks, the exposed porches and dogtrots we have in
our mountain pioneer settlement are of untreated, rough sawn wood.  We find
hemlock to be the best for our purposes -- it lasts for a long time,
doesn't seem to be slippery when wet, and is not very susceptible to the
various insects that enjoy wood.  It works well for the handicapped access
ramps where we need them.

The US National Park Service has a great many boardwalks, footbridges, etc.
in many climates.  The ones I have seen evidently are of treated pine.  For
wheelchair mobility, some have rubberized tracks along the center of the
planks.  Others don't.  I'd suggest contacting the NPS for advice -- or its
Canadian counterpart.  No doubt a search engine would find National Park
Service for you on-line, and an address to which your query could be sent.