Even if you were there, it would be just speculation, since no one can know for sure what is happening into their minds.
You have no idea what they were thinking in that state of mind. There mind was so far gone from reality they could have easily thought that getting rid of the lifevest might help save them as crazy as that sounds. They probably had no grasp of reality when they drifted away
The survivor also described one of the symptoms I mentioned. I have no idea what occurred but I'll hold off on judgment. I'm not a doctor or psychologist.
In all honesty, that sounds peaceful. Drowning sounds so painful, so...scary. I hope they were insane when they died.