And his first or second shot (at most) should have told him to stop.
I'm sorry but that does not follow. The water just means that the shooter cannot see the body, not that he cannot hit it, which makes it even worse. It could still have been hit in the back or legs, as the shooter was aiming down onto it.
A .22 calibre bullet in the back legs or intestines is going to be fatal for a Mink, but not necessarily anywhere near a rapid death. That is why people aim for head shots with e.g. Rabbits, at whatever range, for a clean kill. This is for a good reason. An animal with a shattered pelvis or leg could survive for hours. A poor shot could take the leg clean off, but that would hardly be a quick death!
I am shocked that you say this. Of course there is a 'nice' (right) way and a less nice (wrong) way. A less nice way would be to deliberately target the back end instead of the front. The nice way is to be responsible and humane and GUARANTEE (as good as) a quick kill by making sure you only hit the front end. Pot shots at 110 metres over water where an injured animal cannot be retrieved is blatantly reckless, especially when your first, second and third shots tell you that you are not capable of an accurate hit on your target. I cannot see how any responsible person can defend this, and it is this kind of casual attitude to the welfare of 'vermin' or pest species that gives responsible shooting a bad name.
Can you imagine if it was hit on the raft, but not killed, and then a member of the public who watched this crippled animal writhing around out of reach was told that it was being done to 'help birds'? A wonderful advert for conservation that would be.
The shooter should have known his limits, and even if he tried it he should have stopped after his first shot or two and said "sorry, I can't get an accurate shot at that range".
I'll leave it at that, but hope you can see what I'm getting at.