Watching you travel would indeed defeat the purpose of fast travel. But it would furfill another purpose, more like... relaxing and watching the landscape.
But as you said, there should definitely be a button to press to skip it and actually fast travel if that is done...
Otherwise no would wanna use it I think
It would be pretty boring after a while, but perhaps nice every now and then.

Random ambushes while traveling like that would be even nicer...
Imagine you peacefully watching the landscapes go by as you're sitting on the back of a mammoth. Then a pack of bandits suddenly appear, shooting arrows and using spears/polearms, the mammoth rises on its legs, and you fall off.

It could be done to make really cool scenes I think.

Yeah. I was thinking maybe you heard a loud noise, you look behind yourself, then a dragon is flying directly at you! :ninja:

As one of several means of travel, sure why not. Domesticated and trained mammoths can be made aggressive on demand, explaining why nothing attacks them making them the safest means of travel. When you get to your location, you will even find yourself "well rested", with no hunger, thirst, or hypothermia. It's a fairly expensive ride though (compared to walking). At the start, you may have only one mammoth trip on day, return trip the next. As game progresses you can do quest to improve the service to maybe 4 mammoths having two departures per day either direction.

I doubt that. :shakehead:

Boats are viable, haven't you even seen the (new[er]) map of Skyrim? (alteast on most of the eastern side of Skyrim, if I'm correct that is...)