Up to 80 dinghys have capsized on Strangford Lough in Co Down.
It is understood a regatta was underway as part of the Exe Sails GP14 2014 World Championship at the East Down Yacht Club near Killyleagh when the incident happened, and that around 100 people are in the water.
Belfast Coastguard is co-ordinating the rescue effort and helicopters from Dublin and Anglesea are assisting.
The UK Coastguard says strong winds and squally showers hit the area around 2pm today. Some boats capsized and others were struggling to cope in the conditions.
A search and rescue operation is continuing involving the Coastguard north and south, RNLI lifeboats and the RAF.
Ulster Hospital has declared it a major incident. So far, six people have been brought to hospital.
Strangford Lough is an area affected by powerful tides from the sea. It is a short distance south west of Belfast in Co Down.
It is popular with all sorts of watersports enthusiasts and a dedicated canoe trail has been laid out along its waters.
The South Eastern Health and Social Care Trust in Northern Ireland said ambulance crews were treating as many people as possible at the scene and a major incident response plan has been launched.
“There were 97 people in the water,” the spokeswoman said. “Casualties will be being brought to the Ulster Hospital (Dundonald). A major incident and emergency response plan is in operation.”