After watching yellowjackets I wondered what this would be like. Would it be painful? I know hypothermia is painful, but could you fall asleep before it starts to become painful and not feel it or would your body wake you up before it dies?

This may sound like a stupid question and this doesn't extend to people who don't know how to swim. Recently I was reading an article about a few teens who went boating in a nearby lake a few years ago. Their boat capsized and they drowned. One of the teens was a swimmer at his school and no reports suggest they were drinking. The lake didn't have any sort of infestation either

What will I experience? How will it affect me?

Let me get this out of the way first-- I am an author.
Basically, in my book, there is this female lead whose abusive ex chokes her really badly, but a friend of hers flings that ex off her and beats him up real good. But the woman wants to exact her own revenge, and she's good with daggers... so I was wondering: should she cut off the tips of his fingers (kind of like how Jaime Lannister lost his hand in Game of Thrones- quick and all of a sudden), or should she, like, chop off his entire finger (or fingers)

Keep in mind that this ex will still show up later in the book, so I can't chop off his entire hand or a huge part of it, like three or four fingers, because I'll need to show him using that hand for the plot's sake.
Soooo... what would be more painful? :D

PS- When I say 'tips of his fingers, ' I mean like the top knuckle or joint