Scenic uses a 3rd party map and routing framework. One of the ‘quirks’ of this framework is that it puts a very high penalty on ferry crossings, especially smaller ferries.
Change routing mode to ‘Short’
By changing the routing mode to ‘short’ the routing algorithm will only look at distance, thus ignoring the duration penalty of a ferry. In most cases this will resolve the issue. If not, try this in combination with the next solution.
Drop a via point IN the water
Sometimes you can force the route to take a ferry crossing by dropping a via point IN the water, along the ferry line.
If this doesn’t work for some reason than your options are limited. In these cases you are dealing with a map error. Have a look here to see what your options are in these cases.