it's possible that you were wearing well-insulated shoes

I was barefoot, but I'm also not sure shoes would be that big a component since I was fully in the water?

you just didn't notice the sensation because of adrenaline

I don't remember feeling any shock, and I think I still would have noticed?

twice as fast as it actually goes

Music notation rhythms are relative, so I don't think this has a real meaning?

It's wrong in the sense that it's violating convention for no good reason, and so is hard for people to read. My intended audience is folk musicians who are really used to tunes in 2/2 (and other conventions, like four measures per line), and I'd written in 2/4 by accident.

Your last two Musescore files are missing some separation between 1st and 2nd endings

Thanks! I've uploaded new versions of them that put the repeat sign between the endings, which is also logically where it belongs.

Are child swim lessons common in America? Over here, free swim lessons are now provided for children, and mandatory swim lessons are provided as part of primary school.

It sounds like maybe you're talking about somewhat older kids; is it common for kids in your country to have taken swim lessons by age four?

I would guess the claim afterwards that she couldn't was rationalization, and that the real reason was she was stunned with surprise, just based on the sense that I can't imagine the dress was really that restrictive

The dress was my speculation (and not that it was restrictive, but that it was buoyant under her legs). Her claim was that her legs wouldn't go down, and while I haven't tested this myself in water I think this is probably right: you need to bend your legs to get them under you, and she was keeping them straight due to inexperience with water.

She can't swim yet, no

Maybe even a net positive for her and the family on future water safety.

I don't think so: this could easily have happened when no one was attentive and that would have been a disaster.

I agree it's common, but why is that evidence for it being about CO2?

My understanding had been that it was primarily about avoiding condensation and making it easier to exhale.

Mostly for fun, though I do think having that kind of distance allowed me to be more critical of myself.

you are breathing back in exhaled CO2

Enough to matter?

I would have guessed so, but this one is actually super comfortable.