Ruin Explorer
Legend
Realistically (which D&D is not) wearing any heavy armor and carrying pounds and pounds of metal would be a drowning death sentence. At the very least, countless adventurers would have stripped out of their gear and left it at the bottom of countless subterranean lakes
Not just heavy armour, either - most medium armour would be a very serious issue at best, and hell, I'm pretty sure anyone who wasn't a "strong swimmer" would have very bad problems even in something like studded leather, especially with a few daggers and a longsword strapped to them.
I suspect this is one of those threads best left not pulled at. Flotation aids can solve the whole thing anyway, we know medieval knights in plate got across moats that way, and we know people have been using purpose-made flotation devices since at least several hundred years BC.
http://ravenrescue.com/faqs/the-history-of-the-pfd
If I was going back to a "Did you bring the right equipment?"-style of D&D, well, first off I wouldn't be using D&D for it, I'd use some other system, but yeah, I'd probably look at including flotation aids as part of equipment list.