Keep an eye on this Qwest disclosure spreadsheet to see if any DSLAMs loading on the ferry to the San Juans. I just looked at it, though, and see zilch for San Juan County.

There is a bandit wireless outfit providing service to marinas in Friday and Roche Harbors, maybe elsewhere. Couldn't be certain of your island from the pic or whether that would help. Their quality of service is mixed. Find them here.

And maybe somebody else linked Seattle Wireless' FAQ.
It might have some relevant info. and they used to have good DIY antenna info.