Your
destination is inside land...
But even if you move it in
water you cannot expect the routing module to drive you up to the cat-way
The routing module is intented for relatively long distances where you must take decision concerning
weather. You can put your destination point somewhere close the the entrance of the harbour and probably qtvlm will find a solution.
What you can do otherwise is the create a route manually (route->create) insert a few POIs and you will get a rather precise estimation of the travel duration.
For instance, moving a bit your destination point outside the harbour: