The only time I went fishing I was shocked to see how the fish are killed. I thought it was like you described, and that the fish would die just from being out of the water, but when I reeled the poor, unsuspecting bastard into the boat, the guide took out a massive club and proceeded to repeatedly pound the fish's head until the skull was completely bashed in.
It is interesting that you bring that up. In my experience we would always put the fish in a cooler, or onion sack and they flop around until they die. Sometimes I've been on boats where they gaff the fish, but never where they clubbed them. But, as you say, in other situations, they use various sorts of clubs. Many fisherman who swear by a club actually believe that clubbing them and killing them immediately is actually more humane than letting them suffocate for a couple minutes. Also occasionally a non-clubbed fish can flip himself back into the water in rare circumstances. I'm not sure which I believe is best, but I can't bring myself to club the fish to death. I can just barely bring myself to listen to them flopping around. So that's my decision.