GreyLord
Legend
As a guy who hangs out with people who regularly (not often, but regularly) wear plate mail, I find this unlikely. Most of them, if the armor is properly fitted, could likely swim in the stuff. It wouldn't be pretty, but they could do it. Someone who was clumsy enough that they couldn't stand up in the stuff would be dead meat on a battlefield.
Alternately, in the military I had to swim 50 meters with 30 kilos of gear (more than the listed weight of plate armor in the phb) before they let me out of training. I'd imagine a knight would have an easier time of it, as their armor was more evenly distributed than my kit.
That's because most of their plate armor is made out of modern metal and isn't made like the armor of old.
Originally most knights actually wore chainmail or mail in that fashion. They could move pretty normally. Later plate came about, and at the time made knights almost impregnable to normal damage (yes, their armor was made so that most knights did NOT die from the initial attacks, but later when the stabbers came out on the field of battle and stabbed under the joints). This is one of many reasons for an abnormally high amount of nobles being taken captive from the field rather than having died in the initial battle with their men at arms.
So, weapons evolved to where they could pierce plate armor. At this point plate armor got thicker...to ridiculous levels. They had to withstand gunpowder weapons amongst other items. At this point the armor DID get heavy enough that they couldn't even mount their horses...they could stop bullets...but heaven help them if they fell.
Eventually some would realize that it didn't matter as much, preferring mobility and wearing lighter armor overall, but prior to that, Plate armor got HEAVY.
I don't think I've seen modern plate done for those in the medieval fantasy fighting clubs they have about done in the fashion of those heavy plate armors...probably because most would collapse from their weight in this modern age. Secondly because most prefer to try to recreate the age of fighting with swords and axes instead of polearms, guns, and other weapons in that age of heavy armor.