No, there are multiple ferries and service in summer is frequent! They serve different locations and run several times a day. And there is no Oban-Tobermory (I do not know whether there ever was). The Oban ferry is the principal one; it goes to Craignure and one must drive or bus to Tobermory from there. We didn't use the other ferries, so I cannot rate them, but we enjoyed Oban-Craignure ferry a lot more than the Craignure-Tobermory drive, which was one of Those Roads.

The island in the story is a blend of Mull (the loch) and Iona (no vehicles).