How much weight is too much ? It seems to me that if you were wearing just swim shorts and sandals a small belt pack/pouch and a canteen wouldn't be too much to carry .
IMO it would be worth a little discomfort to carry a few extra things [ especially water and purifiers ] in a potentially very dangerous place like the Amazon jungle.
I personaly would NEVER go hiking anywear unless I was wearing some decent hiking boots/shoes no matter how hot it was . IMO wearing sandals was just asking for trouble .