I took a week long boat ride in the Peruvian Amazon. We did see pink and blue dolphins. The natives said that there were sharks but that they were pretty passive. If you asked about the piranhas they laughed and said they were really no threat, especially in flowing water. So this sounds like mostly hype and for crying out loud, common sense tells you that fresh blood attracks preditors and doesn't draw them away.
Betcha there are crocodiles all up and down the river reading this and thinking, "Dumb sonofabitch".