Actually there were a lot of nasty experiments done during World War II to do with airmen landing in the North Sea, freezing, being re-heated etc, and some other ones done elsewhere to do with rapid excess heat - the conclusions from both were the same - temperature related death is incredibly painful, a lot slower than it appears, and althogether barbaric as it involves massive stimulation of just about every nerve ending in a body BEFORE it affects the brain / heart etc and actually kills. Massive physical trauma (being squashed) is much faster because it destroys vital organs instanantly and is likely a whole lot less painful. Think about it from the fish's point of view, not your own.