When boarding the riverboat to take a long trip be prepared to be cramped for space when you hang your hammock. i got on the boat early to choose a prime space away from everybody but by the time the boat left it made no difference as it was crowded. Bring plenty of snacks with you as the food available leaves lots to be desired.
There are too many VT reports of robbery on Amazon boat trips to ignore. Since I did not lose anything I feel I can make some recommendations for the first time traveler, but perhaps, I was just lucky.
First have as little of value in your luggage as possible. Of course, you should only carry the most necessary of valuables --no jewelry.
Most of what you have of value should be carried on your person. (A under-your-garments pouch is very useful, but also have a more accessible place for spending cash.)
Since I expect that all robbery on the boats consists of someone grabbing something unattended, the pockets in backpacks or suitcases are only good for sunblock, books, toothpaste and the like. Never put a camera, field glasses, a watch, money or important documents in these pockets. This way you can relax a little bit when the boat is docked still remembering that this is the most dangerous time for your luggage.
Still, plan how you will attach or lock your luggage to your spot on the deck. Even just tying it to a pole, or to your hammock can thwart a would be robber. (I actually like to use two methods to secure my bags with one kind of hidden.) Having a way to lock or at least a way to make access to the inside of your luggage time consuming can help protect you from an unguarded moment like when you are eating. If you keep nothing of value inside this is less important, still I generally treat my luggage as if it had something in it I care about. Actually, come to think of it, I usually do have some traveler's checks that I would rather not lose, but since they are fully replaceable I am OK with this.
Lastly for further security, I would strongly suggest you try to connect with the more reputable of your deck neighbors. Usually this would be families with children. They will watch out for you. You can not let fear prevent you from enjoying what can be a great experience with great people on a great river.
River travel is an adventure in itself. The number and variety of boats carrying people and produce up and down the Amazon is fascinating. The boats going to smaller and more remote locations tend to be smaller and provide more primitive conditions. My experience was limited to boats on the main route of Manaus to Santarem and the Santarem to Belem. They can be divided into metal and wood boats. The ticket sales people imply that the metal boats are safer. There is about a 13% difference between first class and second class tickets. Second class is the lowest deck and tends to get less breeze, less light, and more diesel noise and smoke than the second deck. It also has more dockside traffic with loading and unloading.
Rec: 1) Buy first class tickets. 2) Do not pay for one of the small cabins unless you absolutely want the privacy and they have air conditioning that you need. 3) Buy a matrimonial sized hammock -- this gives you extra material to cover yourself or your things. 4) Wear "flip flops", the cheap rubber sandles, once you get on board. (95% of passengers wear them) and put your shoes in your luggage. 5) Carry small bills for possible purchases. 6) When locating your hammock consider where the engine exhaust may exit (right side on one boat I took), the location of the bathrooms, and whether there are tables stored where your hammock will be because your hammock will have to come down for meals. 7) Use sun screen.
The main airport of the Amazonas State is situated in Manaus. Flights from other parts of Brazil including Santarem, Belem, Brasilia, Rio de Janeiro and Sao Paolo as well as international flight from North America arrive here. Smaller aircraft serve the towns within Amazonas. Charter flights are also available.