At the end of last year I spent a very fancy weekend at the Four Rivers Floating Lodge in Koh Kong. Very luxurious!
There are 12 large and luxurious tents, complete with double bed, couches and arm chairs, a very nice bathroom with plenty of hot water, and a small balcony from which you can dive into the river for a swim.
I was too busy swimming and reading to take any photos once we arrived. Sorry.