I use the Android Navigator Ver. 2.1.97 together with OSM maps. There is a routing error when I try to navigate along a ferry. The location is http://www.openstreetmap.org/#map=18/52.49429/12.45347 When you set the start on the east side of the ferry and the target on the west side the program calculates a detour. I checked the OSM and for me it looks OK. The used profile is car and ferry use is allowed.
In 99% of the cases it is the last 50-300 meters of the road that leads you onto the ferry. Those last hundreds of meters are almost always not tagged, or tagged as minor road or tagged as service road.