Joy riders using personal watercraft are putting people at risk on the River Medway, Kent Police have said.
Since April, incidents of anti-social behaviour have seen boats being rammed and yachts being deliberately swamped by large waves.
Incidents have been reported around The Strand and Gillingham pier and marina, where watercraft have also been launched illegally.
Police said they stepped up patrols after receiving up to 40 calls a week.
The Kent Police marine unit has been running operations throughout the summer to tackle the problem.
Police community support officer Michael Dann said: "Since about April/May time, I have been receiving about 30 to 40 calls a week."
He said incidents could be anything from watercraft "slaloming between the moorings of yachts of sailing clubs near The Strand and Gillingham Pier, to boats and yachts having their cockpits swamped with five or six gallons of water cascading over the top".
He said incidents where yachts were swamped led to water damaging the owners' belongings, including electrical equipment.
In one incident, a personal watercraft user tried to swamp another boat, but hit the back of the vessel and caused about £8,000 damage, he added.
Officers said they had received information that the offenders were travelling to Medway from Bexley and London.