Nothing to see here. Move along.
We both know I'd sink like a stone.
Yeah, it's a pretty forgiving rule, considering Labyrinth Lord has chain at 30 lbs (lower than both the original expert rules or Swords & Wizardry but still pretty heavy). But I feel like it actually lines up pretty nicely with the LL rules, which are actually more like guidelines ("likely above" 90% in plate, "as little as" 10% in "lighter" armor, etc). And in addition to being super simple and easy to remember (the kind of rules I like best), it also favors the characters; and as a certified "nice" DM I'm all for little tweaks that increase character survivability, especially since earlier editions are so deadly to begin with. However, keep in mind that characters must make that roll EVERY round or drown instantly. So it's still pretty friggin dangerous to swim in armor, even leather.