Cannabis is a water loving plant, but by no means is it aquatic. It needs regular water/drying cycles to survive. The roots DO require Oxygen, and at tims large amounts. If it stays wet constantly i may be a bad place to grow. Cannabis is very prone to root born diseases, especially when dealing with moisture and rot. Now if he can plant them 'close' to the stream where it stays 'damp' at best it may work. Otherwise he may have to deal with weekly checkups and bringing water to them from the stream in the most severe case.