== Slow boats out of Iquitos ==
You have basically 4 direction out of Iquitos. All these boats includes food. You have to bring your hammock. Possibility to pay more to have a cabin.
1) To Pantoja at the border of Ecuador. Once or twice a month. 80-100 sols (~33$). Between 5-7 days. Stop at Indiana, Mazan, Santa Clotilde
2) To Santa Rosa at the border of Colombia. Every day from Monday to Saturday. 70 Sols (~26$). 2-3 days. Stop at Pevas, Chimbote and Caballococha
3) To Yurimaguas in Peru. 3-4 times a week. 100 Sols (~33$). 2-3 days. Stop at Nauta and Lagunas
4) To Pucallpa in Peru. Monday, Wednesday and Friday. 120 Sols (~33$). 4-5 days. Stop at Requena and Contamana
== Hitchhiking in or out by the river ==
It is maybe possible to hitchhike a boat, but the distance are quite far. Only the regular slow boats are going all the way on big distances. You have also quite a lot of cargo ship. It is worth a try to see if it's doable.