A major sea search was mounted after a number of swimmers failed to heed the clear safety briefing given at the start of the Bournemouth Pier to Pier swim and forgot to check back in.
Three lifeboats and a helicopter were alerted when nine people were reported missing at 2pm on Sunday.
Emergency crews and an onshore search team spent two hours looking for the swimmers before the search was ended. By then, eight were found to have gone home and the ninth confirmed safe at 8pm.
Mounting such a rescue is likely to have cost in excess of £4000 for the helicopter alone.
Portland Coastguard Watch Manager Maddy Davey said they were happy to help : “With nine persons missing in the water, even on an August day, it’s still a hostile environment and if you suffer a cramp when swimming or get swept away it’s very dangerous.”
Clare Pearce, Head of BHF North, West and Wales Events, said: “At all our swimming events we follow a safety protocol which has been agreed with the independent water safety company we work with – Event Water Safety. As part of this protocol we ask that all participants check in after the event.
“We are currently working with the Event Water Safety team to review the check-in process. Any recommendations will be put in place ahead of next year’s events. The BHF Swim Series continues to be incredibly popular and we would like to thank all those who take part and raise valuable money for our fight against heart disease.”
£125,000 will be raised by the 1,800 swimmers participating in this year’s event.