We did the cruise years ago and enjoyed the 23 day experience however it is "Amazon light". We didn't have any trouble with insects but it depends on the time of the year. Do try to take a tour that takes you further down the river. Be very careful while off the ship, we were warned numerous times about not wearing or taking anything of value off the ship. Some did not take heed and had their camera and jewelry ripped from them.
The opera house in Manaus is beautiful but beyond that there is nothing special about the city.