I really liked the Flamingo Hotel. Book in advance and the driver will pick you up at the airport. It's not on the river but it is very close to Walkabout and many other clubs/bars in that area. The hotel is very clean and affordable with decent internet access.
http://www.flamingos.com.kh/
My favorite Phnom Penh experiences:
Sophie's bar
La Cyrcee Bar
Heart of Darkness
Enjoy!