This scam can happen in any tourist city if it happens to be near a river. People offer tours around the city on their boat, seeing the sights. These are often legitimate businesses offering nice city tours.
The con man will set up a stall, near a few boats, that has a lot of foot traffic. The Con Man sells these tours at a very competitive price. After purchasing a ticket the marks are told to come back in a few hours for the tour, and the Con Man points to the boat to meet at.
The people turn up, the Con Man does not. The Con Man just points at a random boat, and hands out self printed tickets.
£10 - £100 depending on local prices.
How to avoid being scammed?
The internet is your friend, you can check for reviews, and see if the ticket seller is legitimate.