@scousemart:
Ideally it would be something similar to the check/uncheck box as described for the Raids may be preceded by air battles in this map.
That would be the most elegant solution, indeed, but not be a trivial change, as that would need to implement a sort of game logic behind it, AFAIK.
An easier workaround would be to somehow alter the space defintions for that seazone in the map itself (xml), for example:
- include a canal definition for the Dardanelles or
- define that zone as impassble
or something like that…
I usually feel challenged to try it out myself when being asked, but real life issues keep me away from offering this to you for a while.
So in the meantime I would recommend you to ask the map-makers on https://forums.triplea-game.org/ for assistance.
Good luck :-)