My idea is that someone will found a city on that very tile that separates land from sea and thereby control the whole river (getting their ships through that city) as well as the only land entrance into the western parts of the continent.
There's a tile like that to the very east too, in the area which separates the large southern Darkfield Peninsula from the rest of the continent. Founding a city on that one, will have your ships going fast between the two halves of the continent and not having to go around that peninsula at all.
Oh. and the zig-zag effect isn't possible in Civ4, unfortunately. It will still have land-units crossing, but keep ships from going through.