I signed up as a pet parent in Steilacoom, Washington but my home is on Anderson Island, a 20-minute ferry ride from Steilacoom. (I wasn’t able to enter Anderson Island as my home location.) Is THS available to me on Anderson Island?
Welcome to the forum @sredman0. All you need do is mention in your Introduction you live on Anderson Island. Applicants will appreciate your clarity on this.
Anderson Island is amazing - I think you should have no problem, if you can be clear where it is and how to get there. 
Welcome! Agree with @temba re:
I’d also add that it would be most helpful if this is the first line in your listing, followed with a brief note that it requires a ferry ride (and where from), for those unfamiliar with the local geography & transport.
On a personal note — I’m a Seattle native and sometimes do summer sits around the region. If you were to link your listing to your forum profile, I’d be delighted to favorite you with an eye toward applying for a future sit if dates mesh up 
. (Alternatively, feel free to DM me here & add your link that way if you like.)
Best of luck!