I’ve seen this behaviour by the Cormorants in Cardiff Bay many times. A few birds gather on the water, then more and more fly in from their various roosting spots to join in. Moving forward together across the water, each bird dives repeatedly.
I assumed the Cormorants were either driving or following a school of fish below the water, and this would seem to be confirmed by a research paper I found online, which discusses a different species of Cormorant living around the Arabian peninsula. Here’s what they have reported:
… data suggest that the benefits of group foraging outweigh the costs of intense aggregation in this seabird. Prey detection and information transmission are facilitated in large groups. Once discovered, shoaling prey are concentrated under the effect of the multitude. Fish school cohesiveness is then disorganized by continuous attacks of diving birds to facilitate prey capture.