Back when I used to fish, and I mostly fished medium-sized fish like carp, taking their head off with a hand axe seemed like the best solution. It’s quick and to the point. If the fish has a bigger head than your axe does, aim for the spinal cord.
I was always too squeamish to do it myself however, which is why I stopped fishing.
My fishing buddy was a bit more hardcore/asshole about it, he’d just grab the fish by their tail and whack their head on the pavement a couple times until they stopped moving. I guess it works too but…way too Neanderthal for my tastes.
Works fine for octopi & calamari though - for some reason I can’t really picture those suffering much and, well, what head are you going to cut off from them anyhow ?! Where does decapitation starts and hacking off their limbs stop ?