Sometimes it is very helpful to change the landscape to permit ships to cross a peninsula. The problem at the moment is that it is too fiddly to create a canal or sea-route using the brush. You try, but the passage is too narrow or not deep enough.
A tool should be included where you are able to "draw a line" on the map, across some land, and that is where a navigable water route will appear.
On a related topic, the same brush ought to be able to "raise the bridge", so that ships can pass under low bridges across water.
