Arrr the ubiquitous Tweety Bream!
Yes they can get down to about 50 ft.
They give cracking bites and love to feed on balled in grounbait.
I?ve actually had takes off bream when they?ve been diving in my swim.
So I don?t think they put feeding fish off. Whether they put off approaching fish is an unanswerable question because you?d never know they were approaching in most circumstance.
What I can say form observations is that they stir the bait up on the bottom, taking some, but can and have done on many occasions stimulated bites not long after they?ve been buggered off??wink, wink, nudge, nudge!
If they really are being a pain you can lead them away from you baited area buy pulting out balls of bait well away from where you?re fishing.
One day I was board and wondered how many balls it would take to fill each bird, so I did a little test and lead them away from my baited area. Once I got them away, I deliberately pulted balls of bait (golfball size) 10 yards in front of them. Each bird, and there were 3, managed 4 balls each and then they swam off full.