Queenstown fishing guide Craig Hind, of X-Stream Charters Fishing, said he spent a night last week on one of his dinghies at Queenstown Bay in the hope of catching the vandals.
One of his spare dinghies, moored at the Queenstown Gardens walking track, had been damaged on Wednesday night.
He witnessed "hoons" drinking at the Queenstown Gardens, stealing oars from another commercial operator and using them to row his dinghy out on to the lake.
They had jumped on the chains to break them and had ripped the side of the dinghy.
"They half sank the thing. It's not worth the money to fix it, so I had to buy a brand new inflatable dinghy," he said.
He had resorted to taking his dinghies away at night and locking them up.
He had notified the police about the incident.
He said a group of young people had boarded another commercial fisherman's dinghy and he had chased them off it on Wednesday afternoon.
It was a problem every summer and security measures like padlocks and chains were not deterring the vandals.
In the past, people had boarded his commercial fishing boat, stolen fuel and had drinking parties.
He said the police needed to step up patrols around the bay.
Queenstown Fishing Guides and Charters had also been the victim of vandals two weeks ago.
Its boat Princess, moored at the wharf, had been untied and its fibreglass had been cracked from being bashed against the wharf.
Queenstown fishing guide Stu Dever said he also had had oars stolen.
His dinghy had been boarded and damaged by vandals in the past two weeks.
He had managed to recover only one of the oars.
Harbourmaster Marty Black said the warm weather had attracted "hundreds" to Queenstown Bay.
It had also attracted vandals and hoons.
He had not seen any vandalism first hand, but had had to "kick" some young people off boats.
"They are swimming out to boats, climbing on and jumping off, but I don't think they are causing much damage," he said.
Constable Hugh O'Reilly, of Queenstown, said he had dealt with a group of Queenstown school children who had gone for a "joyride" on a commercial operator's boat on Wednesday.
"They didn't realise how serious it was.
"They got a bit of a tune-up and were sent on their way," he said.
Boat owners needed to take more care to secure their boats.
People also needed to realise boarding someone else's boat was an arrestable offence, he said.
"It's just a case of an awful lot of drunk people enjoying the hot sunny days, seeing unsecured boats, and thinking they can go for a bit of a ride," he said.
