We went from Belém to Manaus on Amazon Star, and spent a bit more for a "suíte", which is what you see in this video. There is one cabin available on the vessel with a double bunk, however I suspect that it's never available. The "suíte" was fine, the shower is river water, the A/C worked, the ventilation was OK, the bunks OK.
The M/V Rondônia also runs this route and is "bigger and better". The only vessel which might be described as a cruise ship" is the "Iberostar Gran Amazon". We have seen the former heading downriver, however have not seen the latter.
There is also a multitude of older "traditional" vessels, like the Santarem and Amazon Clipper.
We chose the Amazon Star because it was the only one which could take our vehicle as well as ourselves.
Note: there is a tour company called "Amazon Star" which, as far as I can tell, has nothing to do with the vessel "Amazon Star".
If interested, http://suramericacontraelreloj.blogspot.com.co/2011_03_01_archive.html gives some pictures and discussion.