Because it'll be February, and just the afternoon, and short days, I'd walk on. Upon arrival, walk a few blocks to Winslow (town), check out the stores, restaurants, galleries. Walk down to the marina. Return to the ferry via the park trail. The ferry return should be lovely with Seattle skyline lit up at night. Enjoy the ride and Winslow. See this, for example: http://www.bainbridgeisland.com/

Wonderful, thanks for the advice.