The exclusion zone around the G8 summit in County Fermanagh has been extended and boats are prohibited from entering a six-mile stretch of lower Lough Erne.
Ribs, speedboats and a tug boat repainted with blue and yellow police livery are being used to secure the approaches to the venue by water.
Eight world leaders are due to attend the summit on Monday and Tuesday.
Police have warned of potential delays around the airports and in Enniskillen.
The lough restrictions, which came into force at lunchtime on Sunday, will remain in place until Tuesday night.
Boats cannot enter lower Lough Erne from Enniskillen at Portora lock, with the exclusion zone extending north of the summit venue to an area adjacent to Ross Point.
The Police Service of Northern Ireland (PSNI) is patrolling the water and its officers have been joined by several police boats and crews from across the UK.
In a statement, the PSNI said "no form of boat or vessel movement will be permitted within the closure zone, but shoreline activity like land-based fishing will be permitted".
BBC reporter in the area, Julian Fowler, said: "The lock-down on the lough is now in force."
"The police are out in large numbers but so far, only a handful of protesters have arrived in town, with four tents pitched close to Enniskillen Castle."
Speaking from Enniskillen, ahead of the summit, Secretary of State for Northern Ireland, Theresa Villiers, praised the government's decision to host the G8 in County Fermanagh.
"The reason David Cameron decided to bring the G8 to Northern Ireland is because he wanted to show off to the world what a beautiful place it is and how much progress has been made since the political settlement here.
"A few years ago it would have been unthinkable to bring a massive operation like the G8 to County Fermanagh. The fact that it is now happening is a demonstration of the new Northern Ireland," she said.
The Canadian Prime Minister, Stephen Harper, is the first of the G8 leaders to arrive on the island of Ireland ahead of the summit.
Mr Harper is due to meet the Irish Prime Minister, Enda Kenny, in Dublin later on Sunday.
Meanwhile, the Archbishop of York, Dr John Sentamu, hosted a church service as part of the IF campaign to end world hunger, in St Macartin's Cathedral in Enniskillen on Sunday afternoon.
Dr Sentamu sent out a strong message on the issue of tax and said that if there was a halt to major companies avoiding taxes, "millions of people could free themselves from hunger".