#1369420381353676960 kinda similar to this, also I like that one too.
The idea is: You select a target, then a point on / near your territory to choose where the boat starts from, then you can place some markers before finally sending the thing off.
This way, you could tactically move around embargoes of warships and claim new territory more easily.
I would make this a separate button, as the default option to let the game plan the route should stay for taking action as fast as usual.
On trade ship, this could be done the following way:
You can plan trade lines. The routes of trade ships should try to respect trade lines and move along them, if it makes some sense to do so.
Additionally, being able to set warning markers would be nice, so your trade ships dont go into waters full of enemies but stay along your allies, as they highly defer routes passing through danger zones.