I went by car from PNH to Battambang , took 5 hours and was very interesting.
I then did a boat trip $25USD started at 7am and got to Siem Reap at 330pm, apart from very primitive toilet action, it was comfortable.
The trip up the river was facinating, lots of floating villages, lots of people fishing and living on the river.
A real tourist experience.
Much more exciting than flying