Can't help you with the area in question, but I can suggest that you not bother calling the service providers and asking them for an accurate map of coverage. In my experience, they lie through their teeth!
Just as an example, for many years, Rogers and Bell claimed 98% coverage on Manitoulin Island in northern Ontario. In reality, it was closer to 1%.
I would suspect those responders who have been there are correct - would not expect much, if any, coverage in the PNW inside passage. They put cell towers in major population centres, and along major highway corridors. They are not interested in providing service for a few cruisers.