No wonder I spend so much time on this forum

But it works for routing passengers via ferries - given that path costs are time, then you get a overall topology that will show you the fastest way between node A and node B. However, you need to run Dijkstra's for each node in the graph to find the quickest way between any two arbitrary nodes.

SPF algorithms are awesome.

I think he's trying to calculate journey times for ferry crossings, not the shortest route between a set.