Iquitos is approximately at the head of the Amazon which is fed by two good sized rivers: Marañón and Ucayal. Our trip include time on each of the three, (I note that Google maps calls the Marañón the Amazon.) We returned to Iquitos, not Nauta.
We had two guides who were both interesting and very well informed and accompanied us everywhere. The boat crew included 2 waiters and a barman who were also musicians; entertained us every evening before dinner and the wake up calls were singing and guitar playing by them as they wandered about the boat.
As you can see the boat looked a little like the boat in Fitzcarraldo, the fascinating fascinating movie about that was christened SS Molly Aida because of Fitzgerald's/Fitzcarraldo's obsession with opera.
We saw several other abandoned boats that suggested the boat in Fitzcarraldo to me.