[Seabear]

Steve
Decent binoculars are absolutely necessary. Since we were on the Rio Negro the mosquitos weren't much of a bother, the river is more acidic than the Solimoes so, according to our guides, their breeding is limited. But we did take malaria pills as a precaution, and used repellent. You will have more 'squitos, and there are spiders/wasps/etc. The Ex Officio type of clothes with bug repellent built in are good to take.

Our trip was in March, warm during the day, cool in the evenings. Since we weren't harassed by bugs we were able to sit on the upper deck and enjoy a glass of wine each night before dinner. It rained a bit every day, and, of course, was rather humid, but not uncomfortably so. Nights were usually cloudy, we only had one decent sunset.
Most of us had SLR type cameras, you will need a telefoto lens to capture some of the birds, particularly if you come across a hoatxin, they're skittish but really cool.
Our meals were typically beans/rice/grilled fish. Good solid food, we traded sugar and rice along the way for fresh fish. If you like an adult beverage, bring a bottle or two with you from duty-free, it's way less expensive than buying one at the jump-off point. Same with cigars.
Expect an adventure, it's a very different world down there.