Thanks for the link!
I was imagining using the marked sea lanes on the Shogun board. So rather than fleets, armies are allowed to attack along those lines.
I don't have the Shogun board in front of me, but I suspect that if you didn't use fleets you'd have a lot of trouble with stalemate lines. I expect that there are quite a few bottlenecks where you can easily throw enough supports on from one side that it's impossible to break through.
This isn't a problem in Shogun of course because you can just keep building more units to attack with, but in Diplomacy where you can only have one unit per space, I suspect you'll get a lot of stalemate lines.
IMO, I think you'd be better off just drawing some lines for seas and using fleets.