I'll post this on the mechanical board also but I know you all can help.
I've had the water pump changed on my Yamaha F90. It just don't pee as hard as it used to. (I know the feeling). I've taken it back to the shop twice. They agree that it should be peeing harder but cant tell why it isn't. The pump and impeller is new and has been changed twice. The mechanic told me that there must be some kind of build up inside of the engine.

My question is: Is there anything I can run through the engine to clean the water jacket? Is there any way to tell if something is restricting the flow of water? Thanks.