As you get closer to the wharf, the local who has been rowing the boat will show you some goods he has for sale: T-shirts, caps, craftsmanship, etc. He will insist a lot, but you do not have any obligation to buy anything. Just say "no, thank you" as many times as necessary. The man on our boat complained about their low salaries, but I had already paid for the boat tour, and did not find it necessary to buy something I did not need. I did give him a small tip.