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Start by checking your settings.
Fastest route may still be avoiding the ferry.
The ferry may be seen as a toll road. If you have set the device to avoid toll roads, it may well swerve the ferry.
Is the software seeing it as a route (ferry) that has seasonal closures? Once again, check your settings. In short, turn all your preference settings off. Any good?
It might be as simple as the ferry is seen (by BaseCamp) as unroutable. Restart your route on the other shore. Simple but effective.
Still no good? Try creating the route in MyRouteApp or using Open Street Maps, who might see the ferry as routable.