With apologies to the late Charles Stock and the current owner Creeksailor, apparently at this time of year bass can shoal in the river. Certainly this group of seagulls had spotted something interesting and that normally means edible.
They weren't going to be disturbed or diverted by passing boaters either.
I Googled the correct collective noun for a group of seagulls which returned not surprisingly a flock. Also suggested was a squall, a squabble, a rancour and my favourite a sh*t squirt of seagulls.