The RNLI lifeboat from South Queensferry and the South Queensferry Coastguard Rescue Team were deployed, and were only stood down when all swimmers were accounted for.
The event itself was cancelled.
The search and rescue operation followed reports to HM Coastguard that safety boats were struggling to cope with the hazardous conditions.
Kinghorn RNLI lifeboat was brought to immediate readiness to launch as a further precaution, and the Scottish Ambulance Service was also summoned.
Duty commander Mark Rodaway OBE said: “An extensive search of the area was conducted until we were able to ascertain from the organisers that everyone had been accounted for.”
The swim event started from South Queensferry and was due to finish at the old pier at North Queensferry, a distance of 1.4 miles across the estuary.