a couple years ago i dove into a lake at too steep an angle and my bathing suit caught on something at the bottom, and in that moment i knew for certain that i was going to die. i didn't even really have it in me to panic - i was trying to get free, but it was just movement, and underneath that there was just this deep calm of, this is it, and that's ok. and then of course something shifted and i made it back to the surface, and i was okay, but sometimes i think about the fact that i was completely ok with that being the end.
When I almost died of ketoacidosis, I was so sick that I was hallucinating that I was just peeling away into white ash and floating up into nothingness. It was weirdly calm, just like you just said. I thought I'm dying - this is really weird - I'm kind of okay with this. I think the only part I wasn't okay with was that I could hear the emergency nurses/techs whatever they were talking about opening a vein in my neck (to get some extremely-needed insulin and hydration into me via tube; apparently most of my veins had shut down along with my organs so it was hard to get a needle in) and I was so weak and dry-throated that I couldn't make a sound to protest.