CarlManvers2019
Writers Blocked Douchebag
Barring bad geography that doesn't show up on maps, it is actually a good idea.
Nor would (again barring geography) it actually be that hard or complicated. Smallfolk are dirt cheap, you simply have them start digging.
Granted, you wouldn't just do it across the Neck (although that route is quite useful for shipping between Bravvos & Lannisport, and more broadly for reaching the west coast of Westeros). You would also do it in a number of other places, linking rivers/dredging them to make them navigable.
Spoiler of a map that I made for something else
Connect the various bodies of water with relatively small connections and you will drastically decrease transport costs. Travel via water is still one twelfth the cost of even modern land travel; in the age of horse drawn wagons it is multiple orders of magnitude cheaper.
How would they all avoid the flies, lizard lions, diseases and unstable land?
I think this could displace a bunch of Crannogmen too