Good answer, but perhaps you could address the notion of one using a largely or completely wooden shield as an impromptu flotation aid?

I love answers based on actual experience! +1!

I wouldn't dare using a firearm after it has been submerged in water. A shortsword or a light saber should be fine though.

@JohnDvorak, I'd have no worries about using a medieval firearm -- or an AK-47 -- after a swim, as long as I'd kept my powder dry.

Wet leather would shrink, no?

The Olympic swimmer probably don't practice swimming wearing steel armor, so their thrust will probably drop significantly once you put them in armor. It is unlikely the swimming thrust will be consistent since they don't become a motor.

As a data point, I once lifted a bicycle from the bottom of a pond (maybe 10 ft/3 m deep), and brought it to shore. Allowing for some buoyancy from the tires, it might have been 10 lbs/3 kg or so. It was not easy, and I had to do it in several stages...

"This level of fitness takes years of training ... came to exist with the reestablishment of full-time professional athletics" Can you give any pointers on sources that would allow to compare modern athletes with 19th century one? The only stuff I know is first Olympic game records but as I understand it was amateur level people.

I wonder whether the weight would make it feasible for knights to simply hold their breath and walk across the bottom of the river instead.

@Skyler: Not unless it's a pretty narrow river :-) Water resistance makes walking underwater, or even in waist-deep or so water, pretty difficult. That's why you swim head first when diving: to present the least possible resistance.

A large-ish human head weighs about 5 kg. I would have said I can tread water for a prolonged period with my head completely above water. Am I just wrong about this??

@Vashu And there's a reason they were amateurs! There's some signs that professional athletes existed in the antiquity, but since then and till the 20th century, competitive sports mostly existed as a hobby. The competitiveness of amateurs still varies a lot by the sport. Maximizing power in short distance swimming call for building up specific muscles to over twice their normal size, not something one could achieve by casual or job-related swimming.

@JaS One common minimum test for rec dive professionals includes treading water for 15 minutes with their head and preferably hands above the water. And it's easy! The problem is, your head is not free buoyancy - you already need most of it above the water to breathe. Once it starts going underwater a lot, you don't just have to kick harder, you actually lose a lot of your swimming power.

Speaking of the restriction to fresh water - I wonder what the answer would be for someone swimming across the Dead Sea.

Modern military swim qualification includes swims with gear: utilities, boots, flak jackets, helmets & rifles, in total more than 10kg. So swimming in light armor is definitely possible: I've done it, for qual for 50m, and I'm by no means a professional athlete.

@TemporalWolf Good point, but that's a dry weight of 10 kg. Most of these items are barely negative or neutral, having low density and trapping a bit of air. It's the negative buoyancy that really counts, and there a kilogram of steel is very different from a kilogram of nylon and kevlar.

"Steel's density of ~7.75" I take it you mean specific gravity? Specific gravity doesn't need units, but density does.

Static thrust wouldn't be the whole picture. As soon as you are moving through the water, you can get some amount of lift. No matter that the human body is not an aerodynamic (or hydrodynamic) shape, it will be inefficient, but (and I don't have anything at hand to do any math), I would think that it might be possible to lift more weight even at a slow speed than could be supported purely by static thrust. Then there is the question of whether the armor would improve hydrodynamics or just add a whole lot of extra drag along with the weight.

@Therac But surely if I swim mostly with my head submerged and occasionally thrust upwards to get a breath in I could do so with -5 kg buoyancy, which after all is merely the weight I'm saving by keeping my head below water. I would think I could in fact at least a kg or two in addition to this (especially if I'm allowed to have hands in the water to help me thrust upwards!)

@JacobSocolar Swimming with your head partially submerged, and lifting it a bit further out of the water to breathe is already how people normally swim in freestyle. Once negative buoyancy overcomes one's ability to swim in the three fast strokes, the mode you describe - head mostly submerged, using stroke thrust to lift it to breathe - is what they tend to revert to. This is not an efficient way to swim! The body is mostly vertical, and it looks like a struggle.