Solution: I first duplicated the Fields landscape, renamed it Canal and checked the Sea Box. Then I created a duplicate of the Port location and renamed it Canal as well.
I found a suitable Picture of the Nord-Ostsee-Kanal and linked it to the new landscape.
Finally I replaced the Field landscape with the new Canal landscape along the Canal route and added the Canal location type where the road/Railways crossed the Canal.
As you can see from the screenshot, ships may travel between the seas and land units (note the Fallschirm unit)may end their move on the Canal.
Naturally I am quite happy about this 'outside-the-box' solution [:D]
Should be useful for ie the Suez Canal as well.
