I think it should be very simple: if you're in armour, you can't swim in it! At least with the heavy furs you have the ability to shed them.
Furthermore, it is not only possible to get up once being knocked over, you can also do cartwheels, vault (jump) onto the back of a horse, and climb a ladder using only your arms.
I would have agreed with you...until I saw this on Yahoo answers:
It is indeed possible to swim in full Plate Armor, though as one might imagine it would be difficult and exhausting, however if you were in a situation where you needed to do so, not only would it be possible but the adrenaline would give a significant boost.
Exactly.I guess there are also no penalties in the RAW for swimming in lava with armor. 5e is unplayable!
I would have agreed with you...until I saw this on Yahoo answers:
It is indeed possible to swim in full Plate Armor, though as one might imagine it would be difficult and exhausting, however if you were in a situation where you needed to do so, not only would it be possible but the adrenaline would give a significant boost.