Went on retours.eu website and there is a section dedicated to boat trains. These should be examined right now and the only route to have a working boat train service is east coast way using Newhaven Harbour & Newhaven Marine stops. Connection is available to the Cross Channel Ferry to Dieppe. If that was carried out the other possible route is Bahnstrecke Hamburg Lübeck being extended to meet the boat train at Bahnhof Puttgarden for Rødby. Future GEML for Harwich International ferry dock. Which is actually connected to Hollandstrecke Duisburg Oberhausen Arnhem Utrecht via the Utrecht Rotterdam Hoek Van Holland line. This one has the boat train service that goes through Bremen Hbf via Bad Bentheim & Osnabrück Rollbahn
I believe there is a scenario on the East Coastway where you drive a 4-car Bombardier British Rail Southern Class 377/4 on a special boat service from Lewes to Newhaven via Southease. It is called French Connection.
You are correct it is there in the game however other routes can receive this treatment. Example Hauptstrecke Hamburg Lübeck backdated to the 1980s where it terminates at Eisenbahnfahre Puttgarden Rødby Vogelfluglinie. That one is called Bahnstrecke Hamburg Lübeck Puttgarden
