I’ve been on tour with a boat both on inland waterways/lakes and also on the sea.
While routing i.e. with brouter and a river provile works well for rivers and channels, routes trough lakes only work if you stick to the mid line or fairways.
If you like to i.e. cross the mediteran sea from italy to corse, you cannot get any of the known routers prevent the costline but follow i.e. an inland river before.
Also for inland, you cannot route a good way to most habours in i.e. the Müritz (nw of Berlin).
My idea to improve this would be to create additional navigation lines.
Or can we make any use of the coast lines?
Any other ideas?
From where to where?
There are dozens of startingpoints on the Italian coast and dozens of destinationpoints on Corse. Do you map every connection as a single connection? Or do you let them converge to one or two center points somewhere in the sea between Italy and Corse and then fan out again?
And now scale this up to the rest of the Mediteranean or the whole world.
I understand your idea and I think this is ok for fixed signed (by buoys) routes (I made a wiki page for waterway=fairway), but on open sea this kind of navigation is usually not followed. There you take a heading and go from there. Also influences of wind and current can change your path.
For boat navigation, routers need to be able to route in a straight line (or great circle line for large distances) or between waypoints. Use a fairway when there is one, but take a straight line if there isn’t.
Have you seen OpenSeaMap ?