Apparently, saffordpc, the only purpose that the Kiel Canal serves in Diplomacy is that it allows a fleet to enter/exit from either side (no east/west coast nonsense).
I do not see how allowing convoys through Constantinople, Denmark, or Kiel would hurt the defenses of any country in particular. It seems like it would open up possibilities that make the game more interesting, though.
For instance, I'm currently France in a game where I allied with England. I've occupied most of Germany and Italy. England has occupied much of Russia and is starting to press southward, so he needs armies in Russia…
He can do a two-fleet convoy from Edinburgh to St Pete or he can do a three-fleet convoy London to St Pete. However, it is not possible for him to do a four-fleet transfer into Prussia or Livonia, even though in reality these are both much shorter and safer nautical journeys!
If players were allowed these logical convoys I can't think of a single way the game would suffer and I can think of many ways it would benefit.
And let's not forget that this would also add an interesting element to the game: for the three land/water zones (Kiel, Den, Con) it would be possible for an army to break a convoy.
Who do we need to contact to get this rule changed already?!?