I found this piece in a newspaper comment section following on from a none too exciting Nessie article. Apparently it occured around 1979 and involved a "James" from the town of Lewes:
Well I for one believe in the monster. Some 30 years ago I moored my boat on the shallow water shelf at Urquart castle about 100 feet or so from the abyss where the shelf ends and plunges 700 feet or so into eternal night. At dusk there was a huge disturbance in the water just off the edge of the shelf and a flock of 50 or so seagulls were sucked down and all vanished all in less than a second. So yes I believe that there is at least a huge predator living in the loch.
Now whether this was the monster is a debateable point, I have never read of such behaviour in all my time reading the literature. Could it have been a whirlpool? Though I am no expert on how water currents interact, this seems unlikely and is not a phenomenon I have read about at Loch Ness.
Could it have been the actions of underwater gases? Again, one would expect such an event to cause an explosion rather than an implosion of water.
A curious incident for which I have no ready explanation. Whoever you are James, tell me more!