I would have thought whoever 'asked you' should apply for a licence on your behalf including a risk assessment.I have been asked to thin out a population of Tufties on a fishing lake.
As they never come within 12 bore range, is it ok to use a rifle as I do for the Canada Geese??
Legal or not, I'd think any police force hearing of you firing over water would have your licence revoked immediately. On the plus side it's obviously good reason to get a colossal punt gun.I was asking whether it was legal....Not if there was a suitable backstop !!!
The problem is them diving on Anglers baits and becoming hooked which causes mayhem.There is nothing in the legislation to say you cannot shoot wildfowl with a rifle - its not particularly sporting mind. I would question why you want to get rid of lots of tufties as they are not a fish eater like a goosander or other sawbills. They are on the quarry list, but not particularly good eating.
A friend who is a wildlife manager has had a big problem with feral and semi tame mallard and has culled a lot with a 17 HMR - head shots so that they can then be fed to falcons that he also breeds.
Personally, I would try some decoys and decoy them - tufties like to flock and should decoy well within range of a shotgun. Two or three sessions and they will soon get the idea and disappear.
https://en.m.wikipedia.org/wiki/Duck_decoy_(structure)Steel shot is not that dear and is almost as cheap as lead now, if ur gun can handle them.
When there in season u can pretty much shoot them however u want (any safety aspect is up to urself as with any shooting).
Might even be possible to trap them, some old school commercisal fowlers would use large traps/nets, feed them inside some pen sections every day and 1 time shut the gate on them.