I would concur with all the comments. Did this cruise on Prinsendam in November/December 2016.
We did not experience any problems with mosquitoes either on the ship in the river or along coastal areas as the river itself is too acidic (as I recall) for their development. But there are lots and lots of other bugs that will be attracted to the ship's lights; go out in the early morning and see the multitudes of dead bugs on deck.
If you are sensitive to the heat and humidity, be forewarned and forearmed! Someone mentioned 95F temps and 95% humidity and this is not much of an exaggeration. Water, water, water. Light clothing that wicks moisture. Open mesh headgear. You'll need sun protection but not at the expense of being able to efficiently cool yourself so be sure to have performance, sun protective clothing and cover.
One of the best stops on our cruise was, as mentioned above, Devil's Island. You don't actually go to Devil's Island (it's not conducive to tourists on tenders), instead you go to Royal Island which was the administrative headquarters of the penal colony. This is where the solitary unit was located which you may remember from the movie 'Papillon'; the movie did a good job of depicting this area. Be prepared here for long walks up and down ramps and poorly maintained stairs (the latter being built by prisoners) but be on the lookout for monkeys, birds and reptiles. You'll get a very good view of Devil's Island itself on the other side of Royal Island from where you'll dock. Bring euros if you want to spend money at the hotel on the island for soda, water or souvenirs. This is a one-of-a-kind stop.
The river itself is amazing! It is huge, incredibly broad at its mouth (depending on where the ship is on the river and visibility, you'll not be able to see both sides of the river). When you get to the meeting of the waters near Manaus, you'll see a very dark colored Rio Negro merge with the Amazon's milk chocolate color; they run parallel and slowly mix over the course of a few miles down river. Look for the pink dolphin all along the river. Fish for piranha. Get a professional tour guide to take you to places around Santarem where you'll see the way locals have to live in the jungles along the river that floods them out yearly. I found a wonderful guide named Gil Serique (http://www.gilserique.com/) who did a marvelous job with our small private tours, both in Santarem and in Alter do Chao (where he lives and will invite you to stay in his home there!). Manaus is a bustling city deep in the Amazon; be sure to see the theater there. Boca da Valeria is a bit touristy but these people try hard to bring the jungle to the visitors in terms of wildlife and dress.
Do not miss the Boi Bumba show in Parintins! I repeat; don't miss it! It might seem expensive but you'll be missing out on one of the most energetic and thrilling live shows presented only for you (and a couple hundred of your fellow cruisers) by young and talented performers. We didn't know what to expect but we didn't know where the 90 minutes went it was so enthralling. If it's in the convention center in Parintins (think high school auditorium) it will be warm and intimate but you'll want to sit in the front row for the very best views as there is no stadium seating.
It's nice that you haven't got to pay for a Brazilian visa any longer, ours was $160 for 10 years but even this was well worth it. Make the most of your visit before the Amazon burns up!