So even if there is no uncertainty -- they will fail on a 20 -- you still let them roll? From the word "always" it seems like you do.
How do your players respond when they roll their best they can and still fail?
Do you think there's any say to speed up "Okay, please make a roll to swim up the waterfall." <player rolls, adds in bonuses, reports to DM> "You fail" so that it takes the same amount of session time as "You won't be able to swim up the waterfall."
My first response to this was that is an uncharitable read of my post. I'm not a lawyer, and while I'm certain that the lawyers on this forum write with a level of exactitude that mean that you cannot challenge them on an interpretation of what they write, I don't write with that level of precision.
But then I thought, well sure I would let them roll for that.
First of all, I am not in a race, and taking one whole minute to resolve something a player wants to do it not going impact game play.
Context is everything, say describe a cliff and waterfall and one of my players asks if he can swim up it, he has a barbarian after all. I tell him that he does not think it is possible to swim up a 15' waterfall. Let's say that he persists and rolls the dice anyways and it comes up as a critical success. Sure I could just say that he just fails anyways, I didn't ask for the roll and I made it clear that it would not succeed, but players like critical successes so I usually try and do something with it. Like saying that he certainly tries and as the others are watching astonished it looks like he almost does manage to swim up that waterfall before the inevitability of gravity takes over and he splashes back down. I then look around the table and ask if anyone is applauding the effort, we all have a quick laugh, and then move on. Remember that we are all there to have fun and a few jokes and interludes help to break up the adventure and lighten the mood.
Honestly now that I'm thinking about it, this whole scenario seems rather familiar. Pretty sure I did have someone try and swim up a waterfall.
Now of course there is the other side of the coin, sometimes the players are just joking around, other times they misunderstood you. In those cases the player might clarify that they thought they were trying to do something else entirely, we are all humans and misunderstandings are common. The GM not providing a clear enough vision to the players is the usual cause and it is therefore a good idea to backtrack when this is made clear. Perhaps instead the barbarian did a beautiful swan dive into the pool at the base of the waterfall to cool off, then we move on. In any case we manage to continue past the waterfall encounter without it eating them.
On another note, personally I think that very high level barbarians in D&D should be able to swim up a waterfall, or at least do some other sorts of ridiculous, mythic things with crazy high strength. Maybe some day I will finish my rewrite of the D&D 5e classes and subclasses and that can be included....
^2