(Bloody hell here come the number points again. I really don't like doing this you know guys...)
Vietmyke;
1) Humans today are both taller and stronger than humans back then. Both things that can really help with swimming
2) Back then I still believe it would be a rare person indeed who can swim in plate. As for keep themselves afloat was probably quite possible... for a while. But between the exertion needed, the far slower speed due to lack of manoeuvrability and the padding slowly getting soaked and heavier (which there was a lot of as I keep moaning on about) there is no way they could maintain it for any period of time more than 10 minutes, 20 at best.

Summarised: 
Humans today are stronger and taller, better at swimming
Even if they could swim for a little bit back then, they couldn't maintain it for all that long. Want to swim through a river in plate being a medieval person fine, but get knocked into the sea you are as good as dead unless there are others to immediately help you.

Swimming perfectly fine in plate is a misguided opinion, because you aren't just going around in metal and it is very awkward to move in (despite what our good friend IncredibleBee attests to)

Even if you believe that you would have freedom of movement in a suit of plate, it is far from the perfect shape to be swimming in, the human body is quite aqua-dynamic, armour is not