There are several ways you can get around the Seattle Area. The cheapest option (but probably the most time consuming) will be the Seattle Link Light Rail. It is a train that can take all the way from Seattle Tacoma Airport up to Lynnwood, WA (a city next to Edmonds) for less than $10. However the entire trip (if there are no delays) is about an hour. If you plan on doing some sight seeing, or exploring during your stay however, it is a great way to see around the city. 

Here is the link for more info (1 Line) https://www.soundtransit.org/ride-with-us/stations/link-light-rail-stations

The ferry is NOT required to reach Edmonds, WA from either airport. But if you plan on doing some exploring while you’re here, the Ferry is a great way to see the islands of the Puget Sound. Below is the schedule . Keep in mind weekends in the summer are often busier and ferry traffic can be heavy. None of the ferries in the area take reservations.

https://wsdot.com/ferries/schedule/

The Seattle Area Ferries:
Edmonds/Kingston
Mulkiteo/Clinton
Seattle/Bainbridge Island
Seattle/Bremerton Alternate Service