Larne RNLI launched its inshore lifeboat on Monday afternoon following reports of a casualty in the sea at the entrance to Larne Harbour.
The alarm was raised by a woman on the promenade who thought she saw someone fall from a jet ski.
The inshore lifeboat launched at 2.oo pm.
The volunteer lifeboat crew commenced a search from the harbour area making its way north past the promenade to the Black Arch but nothing was found.
Ferris Bay, Browns Bay and Portmuck Harbour were also searched.
The weather conditions at the time were calm with good visibility for searching.
Larne Coastguard was in attendance and in contact with the member of the public who had raised the alarm on the promenade.
An RNLI landrover with two crew onboard also made its way to Carnfunnock and Ballygally slipway to check for a vehicle and trailer, but no evidence of a jet ski launch was found.
Belfast Coastguard stood the lifeboat crew down at 2.40pm following an extensive search.
Larne RNLI Helm Pamela Leitch said: “This was a false alarm with good intent. We would always rather launch and not be needed, than not be called and learn that someone was really in danger. I urge anyone who is concerned that someone may be in distress at sea to call 999 and ask for the Coastguard.”