I free dive and can swim underwater for up to 2 minutes to 15 meters deep, and I do that with little training and live in a desert. I would imagine any self respecting person living near a body of water
back then would outdive me. I mean, I have shit form since I never even learned how to swim. Hell, back then they didn't have all these distracting vidya games and internet to take time away from physical cardio-enducing activities, and free diving is a good way to get food/valuables too.
My character should at least be able to make 30 seconds. AT LEAST. Even you sitting behind your computer could
let your air out, and hold your breath for 30 seconds while still conscious.
Who the fuck can go underwater with tinman gear in reallife / medieval armor and walk up from the water?
Walking underwater is so fun! Have you never grabbed giant rocks and sunk like 5 meters, then walked back to shore? It's really fun... and way more efficient than swimming since you can push off a solid surface to propel yourself.
I kind of wish I had a suit of armor so I could go underwater walking, to prove a point now. It sounds pretty fun tbh.