Please look at a map and follow the run of the Niben River. Then read up on Venice in Middle Ages and Renaissance. Perhaps also some economy 101.
Then explain to me how anybody would get rich of inland river trade.
You are aware that fast travel is only a game play mechanic to make the game more palpable?
You're somewhat ignoring the historical context there. The Nords came from Altmora by the boatloads in the first age, hence they were a fairly good navy. That would also be a bit over 4500 years ago. In this case a good part of the population was seafaring, right now however this doesn't seem to be the case.
Secondly, Ennbjof might be an old reaver but that doesn't connect him to the ship directly because the ship is also from the first Era.
"Thanks for the mazte. Such generosity should be repaid. You know the Nord once ruled this land, don't you? Back when these Dark Elves were squatting around campfires in nix skins? Back in the First Age when the sons of Harald Hand-Free ruled the northern coasts of Tamriel, during the Skyrim Conquests? Well, I bet you didn't know Olmgerd the Outlaw, Harald's bastard, was buried as befits a Nord lord's son, in his ship, in a tomb deep in the mother rock."
They say they buried Olmgerd in the bottom of an ancient Dunmer tomb. From the skald's telling, the burial was on a long finger of land on the southeast coast of Vvardenfell, on a little island close to shore on the west coast of the peninsula. Figure it's somewhere on the stretch between the Daedric ruins at Zaintiraris and Tel Branora. Gods be with you. And thanks for the mazte."
"And of course they buried Stormkiss with him. Stormkiss was Olmgerd's enchanted battle-axe. And you see this old key? See what it says on it? 'Stormkiss.' Here. Take it. Healer says the mazte is killing me. I'm going home to Skyrim to die. You, you got your life ahead of you. Go ahead. Take it." - Ennbjof
While we're at it, the same can be said for the Yokuda navy. Like the Nords both mobilized a good chunk of their population and went too sea. Right now however this isn't the case.