Hey Titan 300...sorry so late...but better late than never.
The main rivers here are the Tonel Sap and the Mekong rivers. Boat is best, I hired a small "ship" for like 100 USD for the day, and went down the Tonel Sap, past the royal palace, and up the Mekong.
There are massive fish here, but the currents are strong. The best is to use local bait, I tried alot of Gulp products, but I doubt those fish know what the hell it is!
There are dams inland that stock MASSIVE fish, mostly some type of catfish, and all you need to do is ask one the the locals how to get to one...Watch out for the commercialized ones, there are not many, but they will rip you off!!!...And go by taxi, the bikes are dangerous off road.