Totally false.
The only thing bongwater is good for is getting away from you and your plants as soon as possible.
Remember, lye (main ingredient in drain cleaner, aka caustic soda) is easily made by mixing water with or draining water through wood (cellulose) ashes. Cannabis is also mainly cellulose; when you burn it in a bong, the ash lands in the bongwater.
How much drain cleaner would you put on your plants?