One thing definitely worth bearing in mind if you are swimming anywhere in the Amazon is that piranhas and stingrays (raia) are common. Piranhas are actually much less of a problem than you would expect. Forget any films you have seen; they don’t attack in shoals, prefer still water to currents and no death or serious injury from piranha attack is on record. Nevertheless, they can give you a nasty bite and are indeed attracted to blood. They frequent particular spots, which locals all know about and avoid, so ask for advice.
Stingrays are more of a problem. They love warm, shallow water and are so well camouflaged that they are practically invisible. If you tread on one, it will whip its sting into your ankle causing a deep gash and agonizing pain for at least 24 hours. However, stingrays really hate noise, crowds, waves and strong currents, and so are rarely found on regularly used beaches, such as Alter do Chão, near Santarém. But off the beaten track, they are an ever-present threat. You can minimize the danger by wearing canvas boots or trainers and by splashing and throwing sand and stones into shallow water if you intend to swim there.