I took a taxi out to Roche Harbor and didn't know that there are NO published rates on Friday Harbor, so they charged me $35.00 - two people, 2 bags. Unreal. Turns out there is another taxi that only charges $20.00 for the same trip, so be sure you find out the fare before you get into that cab!
Unique Suggestions: Find out the fare before you get in, shop around.
Fun Alternatives: The "transit" bus runs, more or less on the hour, $5.00 each but they weren't there when my ferry docked.
Watch out for whalewatch trips that are less than 4 hours. You need this time to get out to the whales and actually spend time with them. There are some companies that offer cheap tickets, but you will NOT get the same experience.