Abstract: An on-board system and method for treating lakes and ponds with chemicals is disclosed. The system is contained on a watercraft and can operate continuously without interruption. Water is pumped on board from the pond or lake and strained. Treatment product is introduced into the flow stream by a controlled mixing means to form a prescribed treatment solution, which is then delivered into the pond or lake. The mixing means includes a Y fitting coupled upstream from the pump. In another application, an inductor is provided downstream from the pump for mixing solid product with the flow stream to form a treatment mixture. The solution or mixture, whichever the case may be, is discharged into the pond or lake via a boom and/or a hand line outfitted with a nozzle(s). A conduit system with pump(s), valves, and manifolds connects the intake and the discharge ends of the system.