Police have warned youngsters to stop diving off piers and boats in Harwich harbour.
They fear that someone will be seriously injured - and are also concerned about the damage being done to fishermen's boats.
Sgt Peter Wood said a number of incidents had been reported.
"These youngsters are jumping and diving off Trinity Pier and the old railway pier which is very dangerous in itself," he said.
"However, they are also swimming out to ten or so boats in the harbour, clambering aboard, getting onto the roof and diving off there as well."
Sgt Wood said equipment such as aerials on the boats was being damaged and items had also been stolen.
He added: "I just hope we catch who is responsible before the fishermen do. This is affecting their livelihoods and they are more than a little angry about it."
Anyone with information should contact Harwich police on 01255 241312 or Crimestoppers on 0800 555111
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