DND_Reborn
The High Aldwin
Personally, I disagree. Climbing 80 feet of rope, even knotted, is a difficult situation. Most people could not, or would not, even do it. It would require quite a bit of strength and endurance. And most people, I would argue even heroes, wouldn't be able to move 30 feet up in just 6 seconds, especially when most PCs carry 40-80 lbs of gear.It feels like there's a lot of legacy thinking going on here i.e. climb = "climb check." That's not how it works in this game. Climbing is just a matter of a hit to a character's speed unless there's a difficult situation that complicates the climb. As laid out by the OP, there were no such difficult situations in play.

But hey, you run your game your way. I'll have the PCs make a check, because up to an average of 70 points of damage is significant.
And FWIW, your thinking would almost be a kin to saying: "Well, the monster can only hit the PC with a 20, so I won't bother rolling for it."
Now, I am not arguing your point of the intent of how 5E works. As others have said, if you are just swimming, but the waters are calm, no real risk of drowning, and the distance is not great, then no check. But if the PCs have to swim a mile or something, yeah, so sort of check is required, even if it is easy, because drowning is definitely possible.
Maybe you are the type of player who doesn't consider such mundane threats as worthy of the time in a fantasy game? No problem, that's cool if true, but again I'll be requiring a check because otherwise I am just removing a source of tension in my game. Just my preference. shrug