I had a friend who asked me almost exactly this on the way into the pool, shortly before she got into some trouble herself.
She has lung problems, is not a strong swimmer, and had not been in a pool in awhile–possibly a couple of years. She’d had surgery on one lung about six months before, which had reduced her already compromised capacity.
So, less than five minutes after asking me, “How can people drown, anyway? If you get into trouble you can always just float,” she jumped into the water, the squeeze of it gave her an attack of breathless, the diminished lung capacity & everything at once made her gasp, she inhaled some water, and she came up with a panicked look on her face.
Now I was once a qualified lifeguard. (In the 1960s. For more than 20 minutes, though. But still, a long, long time ago.) I could see she was in trouble and, while I am a strong swimmer, I didn’t really want to test my extremely rusty lifesaving skills (the only thing I could remember anyway was, “don’t let them drag you under too”). So I did not swim over to her but got behind her and guided her to the rope, and then when the lifeguard helpfully yelled not to hang onto the rope I swam her over to the ladder (a distance of about 8 feet, which we covered pretty slowly). While she hung onto the ladder I climbed out, got her inhaler out of her bag and brought it over to her and by then, the lifeguard had decided to come over and see if there might be a problem. Well, if my friend had been swimming by herself there might indeed have been a problem. Although I think she could have made it to the rope, and I’ll bet she would have hung onto it despite what the lifeguard said.
So a few minutes later, when we were in the shallow end and she was recovering slowly, I said, “So why didn’t you just float?” She said, “Well, it’s hard to just float when you can’t [expl. deleted] breathe.”
Okay. So that’s how people can drown.
Also contrary to folk wisdom, drowners don’t necessarily go down three times. (I’m sure I don’t have to mention this here, but I am anyway.) Once will do it. The two people I have rescued (both children, neither in an official lifeguard capacity, and I am not counting my friend) were both overconfident and they MIGHT have come up on their own but I figured why risk it and hauled them up myself.