That's a popular activity in the Pacific Northwest.
From Anacortes, Washington, you can ferry out to the San Juan Islands. Spend a day per island, and you can easily spend 4 to 5 days in the San Juans.
You can than ferry over to syndney, B.C. (Victoria) , and then ride up Vancouver Island, ferry over to the Gulf Islands. Keep going north on Vancouver Island, you can ferry over to the Sunshine Coast. Then ride back to Vancouver...
And so on...