I've been obsessed with the "swimming in armor" rules for a while. Every system has a say on it, with varying degrees of complexity, but none of them really strike home with me. They run the gamut from the not-very-realistic to the not-realistic-at-all. With Dungeon Crawl Classics they just didn't want to tackle this issue, I guess because of the inherent difficulty. Finally, inspired by the extraordinary Fire and Brimstone! A comprehensive guide to lava, magma and superheated rock (Link!) I think I've managed to design an elegant yet simple rule that can be used not only with DCC, but also with any Dungeons & Dragons clone or iteration:
If you fall into water, you're completely covered by it, and you're wearing armor, you'll sink to the bottom and you'll drown. Yes, that's right, even if you've got Strength 24. If you get out of the armor before drowning maybe you'll be lucky enough to survive, but don't count on it.
What's nice about this rule is that it can also be applied to characters wearing heavy backpacks, cement shoes, or handbags with a brick in them.