I live at the seaside Lord and we have thousands of them, they wake me up at dawn every day, give me a crossbow and I would enjoy my mornings hanging out the window for an hour or two.
I have never seen a Herring Gull actually dive for fish or a Black Headed for that matter. There is a Tern colony near where I fish and I do see the occasional one with small fry in its beak as Peter said. However, Terns are far less common than Gulls and are quite a shy bird who nest far from humans mostly. I doubt they will adapt to human civilization as Gulls have done so, I doubt they will become a threat.