Travel
We highly recommend this guide:
https://www.visitsanjuans.com/getting-here
From Sea-Tac airport, you have 2 main options, the first option is to drive north or take a shuttle to Anacortes and take the ferry to Friday Harbor. This is how most people get to the island. We highly recommend that you make a reservation for the ferry if you’re driving, which you can do on this site:
https://wsdot.wa.gov/travel/washington-state-ferries
One important thing to note is that they release 30% of tickets for reservation on April 25th (2 months before season start date), another 30% 2 weeks before the sail date, and another 30% 2 days before the sail date. You can also get there early and wait in line (if you do this, avoid the final departure of the day).
Another option is to take a flight from Seattle to San Juan Island:
https://www.visitsanjuans.com/airlines-charters
There are other options to get to San Juan Island through Port Townsend, WA and Sidney, British Columbia utilizing the Ferry.