I think you're right.After thinking about it a little more, it probably wouldn't be that useful a tile. Roads already score less than cities and don't tend to be big scorers unless you have an inn or are working on a particularly big road. Unless the 'flooded road' tile happened to be drawn when someone was doing that, it probably wouldn't be that useful. Put it on any normal road and the player would simple close the road at the earliest opportunity and start a new one, with no more than mild irritation.
I just particularly like siege tiles, and I also use roads a lot, so anything which combined them would be super! But you're right, it probably just wouldn't work.