Pain and death are horrible because of your level of awareness of them.
When you stub your toe, the pain is intense. It's a brilliant flash that you'd rather avoid. It can hurt worse than being shot - but being shot has a lot more emotional response associated with it. There's the aggression involved, violence, and of course fear of death or injury. It's the fear of what that pain means that makes it agony, that and the hardware to fully experience it rather than just register and react - more so than pain itself.
I'm pretty damned sure that a fish can be overloaded with pain signals. If you started gutting it alive, it would definitely feel it. But be honest - it wouldn't experience it in the same way that a lizard, or even a frog, would. Fish can display a basic mechanical intelligence, but the thing has no consciousness in a meaningful sense - it doesn't experience the world, it just exists in it. Any pain it feels is along the lines of a stubbed toe - it's reactions are out of reflex, there's no depth.
Do we know a fish has no mind? No, not with any certainty. Maybe everything is aware on some level, and it's wrong to kill anything for your own sustenance, etc etc. But... I don't believe in mysticism - a primitive animal is a primitive animal, and I'm not going to shed any tears for what happens to them. That changes as you go up the ladder, though. I have concern for a cow or a pig's quality of life, but I'm not remotely sorry for their death. I just wouldn't want them to suffer for it, because they're mammals (to be blunt about it). Chickens, I'm not as concerned about, within reason. Reptiles, I give the benefit of the doubt, and same with amphibians - though more grudgingly. Fish and anything lower, as far as I'm concerned, have no "well-being" to worry about.
A crab or lobster is so far down the totem pole that I don't think concepts like misery or torture can apply to them in any form. They react to pain, but that pain does not have the same meaning as when we're talking about something with an actual mind.
Tossing them in boiling water looks gruesome, but it's effective. All this talk of a quick kill ignores the fact that total removal of the head doesn't do a whole lot. You boil them because poison or impact trauma are about the only other alternatives. If you want to be more humanitarian, electrocute them. It's a little like worrying about offending a cockroach though - there's no common-sense in it.