What an enormous headache:
No doubt, there are details involved in the above that I hadn't considered. But the nice thing about a wiki page is that it can always be improved.
The reader might wonder why I would take so long to outline what most games would consider a very simple situation. Most DMs, no doubt, would be ready with a boat whenever the party got to a river, as the main thing is to get the party across and off to the adventure. Who in the hell wants to sit around waiting for a boat when there are dungeons to be plumbed?
I see the question in the same way that encumbrance is viewed. In reality, boats aren't waiting conveniently for party members. What's more, in a wide and complex world, there are dozens of different circumstances to account for. Obviously, it's easier to find a boat in a big city than it is at some obscure back-country cart track dead-ending in a river like the Mississippi or the Loire. I've tried to account for that in a way that's predictable for a party familiar with the area, but very difficult for complete strangers.
Consider the effects of arriving, by surprise, at a river too wide to cross safely. Yet there's a small dock here, and a few souls around in the "thorp" to tell us when the next boat is expected. They tell us it'll be 26 hours. It's four in the afternoon now, on a Wednesday, so that means, what, Friday noon? Thereabouts, given that the boat might be early or late. Do we go back to town and try a different route? Do we wait? If we ask the locals about going following the river in one direction or the other, what do we think if they say the next place across is six miles down river? What if it's 12 miles, or 20? Do we wait? Do we go?
And if the boat won't stop where we want, do we wait for another boat? Do we act as pirates, seizing the boat for our own purposes, hoping that if we give it back to the boathandler, he'll forgive us?
These questions may seem mundane, but a party will argue about it vociferously if given the opportunity. Because they'll care what they do. Yes, they may hate the situation; they may wish there was a game-story boat waiting for them, but there's a bigger gain to be made rather than assuaging their immediate wants.
Whatever they decide, they'll be in control. Do not underestimate the emotional effect of this. There's something soul-crushing about knowing that you're constantly in the DM's charge as he or she shuttles you from place to place, without your freedom to make the decision. If you go back, or wait, or follow the river, the DM has to adjust to YOUR choices. YOU'RE in control here, not the DM. Over time, with multiple situations beginning with boat docks and reaching to much larager facilities, this provides a feeling of ADVENTURING that being shuttled across the river to the next prefabricated dungeon can't provide.
It's hard to grasp that for a lot of DMs ... especially if they're the sort that's moving the party along because it's the DM who hates having to wait at the river more than the party does. Believe me, that's a thing. More often than we'd like to admit, the momentum of a game is based less on what the party wants than what the DM wants the party to want.
You can detect this rather obviously if your DM ever says impatiently, "Make up your minds, dammit!" Hm. If the party isn't in such a gawddamned rush, why is the DM?
Is it because he or she knows everything already, and is bored by the unpleasantness of the players enjoying the game world?