I have seen that mr_siika made one some time ago, but nobody ever used it... I wonder why.
It's a great concept, but the reality of things is that it just wouldn't work out very well. You can only move something so fast before the game engine chokes on it and dies. There's also the small matter of encounters on the roads. It would be nearly impossible to suppress that without causing a whole bag full of compatibility problems. That's probably why nobody has made use of silka's cart, and probably similar logic behind why Bethesda didn't do something like it with Morrowind.
I thought about adding boat options but it really isn't practical to go to all the trouble for two destinations. So I'm glad the design decision turned out to be something others like too.