From the way I see it, most of the citizens of Fishman Island are idiots. First, let's start with the three unwitting mermaids who got themselves caught by Caribou. They think the barrel contains alcohol, but does a barrel containing alcohol needs to be tightly sealed that you need a crowbar to open it? I guess Franky should have put a sign saying, "DANGER, DO NOT OPEN". The other mermaids knew the kidnapped mermaids were opening the barrel and they should see that the barrel has been opened and emptied and they still don't suspect a thing. And yes, the citizens went on to suspect the Straw Hats and not what was inside the barrel. Then after seeing Shirahoshi, the locals quickly came up with the conclusion that Luffy is kidnapping the mermaid princess. The locals saw Luffy beat up Decken to protect her and they saw the princess went with Luffy voluntarily and they still think that Luffy is kidnapping her. One of the Fishman Island citizens even came up with the theory that Luffy is going to sell Shirahoshi to the World Nobles. I mean come on, Luffy beat the crap out of one of those nobles and saved a mermaid (Keimi) from slavery. Why in the world would Luffy sell the princess to them? I wish that someone would beat some sense into those ignorant locals. And I hope either Luffy or Sanji gives Caribou a painful makeover.