Eh, drowning is probably the biggest suffering you can create without going for extra effort. Just because it's quiet doesn't mean it's fast, I'd take being beaten to death over being drowned any day.

Besides, as I said, if you are not able to kill something quickly, you are not qualified to kill it. People who do stuff like catch fish and then leave them to suffocate just piss me off. If you don't have the ability to at the very least bash it over the head, then get someone to do it for you.