[AndrewC]

I hear that drowning, or oxygen deprivation is a pleasant experience.

It depends. Hypoxia can be insidious so it does not always raise red flags. I can say that it is not unpleasant for me, at least as experienced in an altitude chamber.

[Nebullis]

Hypoxia can be insidious so it does not always raise red flags.

That's true. The out of body or near death experience is a sure tip-off, but not everybody will experience that. And for some people a delusion is not recognized as such.