A major rescue operation has taken place in Strangford Lough, where around a dozen dinghies were overturned by a wave in stormy weather.
It was confirmed that 20 people - including children - were stranded in the water at Killyleagh where an international boating event was taking place.
The Maritime and Coastguard Agency said 10 people were injured.
Among the injured were two teenage boys, a woman, believed to be in her 30s, who injured an arm, and another sailor who suffered a head injury.
It is understood one person was in the water for about 20 minutes and most of those who ended up in the water were treated for the effects of hypothermia.
Ambulance crews were treating them at the lough.
"It's believed that all those in the water have been accounted for," said a spokesman.
"However rescue units on scene, including the helicopters and lifeboats, are carrying out further searches to make sure."
Initial reports from several official sources had said up to 100 people may have been in the water.
However the coastguard has since confirmed 20 people went overboard.
A spokesman for the East Down Yacht Club said two competitors suffered suspected broken limbs.
He said no-one else was seriously injured while a number of sailors suffered nothing more than cuts, scratches and bruises "consistent with the sport at this level".
The father of one of the sailors out on the lough said it was a "really worrying" time for him as he waited for news.
"You dont know what to expect, everything goes through your mind," said Alex Porter.
The Bangor and Portaferry Coastguard Rescue Teams, the Portaferry and Newcastle RNLI lifeboats, the Irish Coast Guard helicopter along with the helicopter from RAF Valley were all scrambled to the scene.
The Ulster Hospital in Dundonald declared a "major incident" with staff on standby to treat casualties. They have now been stood down.
A Northern Ireland Ambulance Service spokesperson said: "At 2.15pm this afternoon the Coastguard contacted the Ambulance Service to declare a major incident as a result of possibly up to 100 people in the water.
"We have sent a number of officers, four A&E crews and three patient care service vehicles."
The Exe Sails GP14 2014 World Championship are taking place at East Down Yacht Club this week. The event sees one of the biggest fleets of single class twin crew dinghies assemble in Northern Ireland.
Competitors were on their way back to shore when the incident happened.
A spokesperson for the event said the first race had been due to take place at 11.55am this morning, but it had been cancelled due to bad weather.
Following the rescue, organisers said boats had begun to head ashore when a "strong squall of 31 knots" passed over the race area.
"The effect of this was that some of the GP14 boats capsized - this is not an unusual situation and crews are trained on how to 'right' their boast, " a statement said.
"Unfortunately a further squall registering 37 knots followed the first, capsizing a further number of the fleet."
The Coastguard today said: "Belfast Coastguard was first contacted just before 2pm reporting that some of the boats had capsized, while others were struggling to cope in the strong winds and squally showers."
Ulster Unionist leader Mike Nesbitt praised the efforts of the emergency services.
"It was people of all ages, and some very high winds presented themselves. A lot of the boats were ditched into the water," he said.
"It's very cold out there, all credit to the emergency services.
"Some people have minor injuries. some have some signs of hypothermia - but the situation, thank God, appears to be under control."
One of the competitors, Tom Daniel, 20, from Halifax in Yorkshire, said his boat overturned twice.
"A massive squall came through, a lot of boats went over. We got very wet, a few got various knocks and bumps going over but nothing serious," he said.
"A bit further along the line a lot of people went in because a massive squall came in and we ended up back in.
"We thought somebody was seriously hurt. We thought there was more damage than there was because a few people did look like they were struggling but it is part of the sport and this would not normally attract too much attention but people from the shore saw that there was damage and it has been blown out of proportion."
Mr Daniel said the wind was not extreme.
"The guys coped with it very well but some people did struggle a bit," he said.