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.
Drop a via point IN the water
In 95% of all cases 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.