If you are going to kill a puppy for no good reason, drowning is a relatively humane way to do it. Once you're at the killing a puppy for fun stage in your decisionmaking, most of the fucked up has already happened. Drowning doesn't make it more fucked up.

It's not like someone is gonna say "you killed that puppy for no reason. But you really crossed the line with drowning."

I can think of many cases where drowning would be a preferable alternative to other modalities of needless puppy murder.