personally my experience of live wild animals caught in traps tells me this guy probably made the best choice given the situation, not saying it was morally right or wrong just if I was in the same situation I'd probably do the same, I've had enough experience with cornered wild animals to know even the small cute furry ones can be vicious little demons. that said if I was setting traps I'd atleast have my rifle with me while checking them especially if the trap was designed to catch them alive.
to be honnest i agree with the RSPCA in general as even as a hunter I do not like to see animals suffer but that said drownding takes all of a minuet at most, which is more humane than some methods I've heard people use.
one personal example years ago I used to raise turkeys for christmas dinner, one year we had 3or 4 left over because people had cancled their orders so we figured we'd keep them and kill them for their meat through the year. 3 months down the line they all get an infection resulting in their heads swelling up with 3-4 lumps each (one was so swollen he couldn't lift his head) the nearest vet to where I live is 30-45 minuets away depending on traffic and since we don't have a car that's not an option so we decided to put them down, my first attempt was to shoot them with teh rifle. even after 4 shots at point blank range with the rifle it was still alive. well I quickly realised shooting them wasn't working and would probably cause more suffering (assuming it was suffering as it continued to eat it's food as I put the rifle down) I then grabbed my knife and decided that was the better option (luckly I keep my knife very sharp) taking their heads off with one clean cut each, granted the RSPCA would probably have some words to say about that since they appear to be opposed to anything more harsh than petting an animal but in my eyes sometime the RSPCA approved method isn't always the best, and given the option between letting an animal suffer needlesly or dispatching it with whatever I have available I'd take the latter no matter what that entales.
but those are just my oppinions