It's an interesting question whether it's *possible* to swim in chain mail. (I know it's mentioned in Beowulf, but 'cmon, it was a boasting contest to begin with!)

I've worn (and fought in with SCA-legal armor) chain mail--40 pounds is about right. In my competitive swimming and lifeguarding days, I would probrably be able to doggy-paddle with chain mail on, or tread water, but not much more. Also, I'm a woman, so I do have some--ahem--natural advantage.

I'll put out there that I would suggest it's *possible* that a Northman, out at sea, for some reason wearing chain mail in a ship going down, could survive plunging to the depths of the sea. The Scandinavian cultures, unlike much of Europe, were documented as embracing swimming as both a recreational and survival activity. Plus, you *can* get out of chain mail fairly easily in the water if you get yourself facing downward--but granted in a survival situation there's an element there that's a bit risky.