Foolhardy youngsters are putting lives at risk by jumping into Whitby harbour in front of yachts and dinghies.
Now boatowners are calling for action because they fear there will be a tragedy.
Bryan Clarkson, skipper of the Skylark, which takes parties of holidaymakers out to sea, said "The problem is getting worse."
He said that one yachtsman had almost capsized when a group jumped off the fish pier together, creating a big wave effect.
Others have complained of eggs being thrown at their vessels.
Whitby Yacht Club has written to Scarborough Borough Council, which is responsible for the harbour, saying that something must be done.
It points out that the youngsters are jumping from the piers, swing bridge and yacht club pontoons.
Harbourmaster Bill Estill said "We have given stern warnings but the problems are continuing.
"We are doing our best to warn the teenagers away and telling them of the acute danger they are putting themselves and others in.
"If one of them was hit by a passing boat there would certainly be a tragedy."
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article