The captain of Carrickfergus Sailing Club has said the fire which destroyed the building is thought to have been started by an electrical fault.
Fire crews were called to the building on Rodgers Quay shortly after 23:30 BST on Friday.
A few members of the club were inside at the time and tried to put out the initial fire which is believed to have started upstairs. No-one was injured.
The club has been at the site for 35 years.
Because of the strong winds on Friday night, the fire spread quickly.
As a result firefighters concentrated on stopping it spreading to other nearby buildings.
Club members have been holding an emergency meeting at the marina on Saturday morning.
Club captain Debi Hetherington was at the clubhouse shortly before the fire broke out.
"This morning at eight o'clock I went and put the radio on and heard the news and I just could not believe what the news people were saying," she said.
"It's just horror - shock, horror."
East Antrim DUP MLA David Hilditch said it was a huge blow for the town.
"The sailing club is an integral part of Carrickfergus," he said.
"There's also a number of associated businesses there and indeed a family franchise restaurant within the complex.
"It was a very popular location, so this has been a major blow this morning for Carrickfergus."
Sean Neeson, of the Alliance Party, said the important thing was that no-one had been injured.
"Thankfully nobody has been hurt - you can replace the building, but you can't replace people," he said.