Swimmers off the coast of Dorset were ordered out of the water when one beachgoer raised the alarm over a suspected shark.
Lifeguards put up red flags to warn people, saying there had been sightings of “large marine wildlife”, witnesses reported.
One beachgoer said an announcement was made over a megaphone to tell people not to swim.
The lifeguards searched the area for half an hour but could not find anything, the Bournemouth Echo reported.
The RNLI told the paper: “Lifeguards flagged the beaches for a short period of time to investigate reports received of unidentified marine life.
“We still have no confirmation of what the creature was, and everyone going into the sea should enjoy it but, as always, respect the water and exercise an appropriate level of caution, including keeping around 200 metres from marine life for your and their safety."
The beach was reopened more than half an hour later.
Even if it had been a shark, the danger would have been minimal. Shark attacks are extremely rare, experts say, and people are more likely to be struck by lightning than attacked by one of the fish.
Watch: Birds eye view shows sharks swimming through schools of fish