There are no restrictions on naval movement like that seen in FMB as the mission file is created directly by DCS.
Why someone plotted a Kubelwagon to drive across the water is interesting but an obvious don't do that type of thing!
Hmmm, must test that as I would think it should sink when it hit the water but anyway people shouldn't plot that sort of thing should they?
The positives of this of course is that ships can be plotted to go into harbours, small boats can be plotted along rivers etc. Places they normally can't go using FMB.
The thing to look out for is that the map in the MP has enough detail to plot these moves accurately.
You can otherwise (to navigate a tight course) get the co-ordinates using FMB that you would like the ship to follow and edit the X & Y of each waypoint while plotting in the Mission Planner. This results in a very acurate plot that does look good in mission in tight places like rivers and harbours etc.
Basic rule is plot ships to give the land a big miss where possible and edit starting and final waypoints for fine course detail if needed.