Troublemakers at the Olympic 2012 rowing venue are being warned they face a criminal record or serious injury after a number of recent offences.
Staff at Dorney Lake, on the Berkshire-Buckinghamshire border near Windsor, have reported illegal swimming in the private lake and criminal damage.
Police will be making surprise visits to the area over the next few months.
The 2km (1.2-mile) stretch of water will be the venue for the rowing and flat-water canoeing regattas.
Sgt Dave Bryan said: "If we catch anyone who shouldn't be there, we will deal with them appropriately.
'Risk of drowning'
"A conviction for criminal damage is very serious and, apart from facing a prison sentence, will mean you won't be able to get a visa to visit countries like America and Australia."
Ivor Lloyd, managing director of Dorney Lake Society Services, said: "This is a professional rowing lake on private property, with a total of 18km of submerged cables.
"Swimmers who get caught up in the cables risk serious injury or drowning, so please stay out of it before an illicit swim ends in tragedy."
Construction work at the venue was completed on time and on budget earlier this year.
About 30,000 spectators a day are expected to converge on the area for the rowing events.