We have had a few encounters with what appeared to be shady characters but none that developed into anything serious. One of the funniest, at the time or perhaps you had to be there, happened shortly after we entered Regent's Canal after coming down the Thames in 2004. Nearing a lock 3 or 4 youths stood watching as we approached. I was driving and my wife, having walked from the previous lock, appeared a little nervous. Not to be daunted she asked the lads if they would like to help work the lock. After a bit of stunned silence they replied "yeah, orright". I chatted to one as the lock filled and he was fascinated by the locking system. He said he had watched a few boats going through but had never before been asked to help. He even offered me a "smoke" which I politely declined.
We have found that if you engage people in conversation it seems to put any misdeeds they may have been contemplating out of their mind.
Cheers
Bill