A manual hand pump head has a through the hull connection below the water line to allow the sea or lake water to be used for flushing the head. By pumping the lever with the indicator in the flush setting the water is drawn in from the outside to flush the head. To flush the water out of the head, set the indicator to the dry setting to flush from the head to the holding tank. One unavoidable issue with these systems is the microbial animal life in sea water will get trapped in the line leading from the hull to the head. The sea life in the line will die and the first few pumps of the lever can leave the head with some very foul-smelling water. When that happens, quickly switch the indicator to dry and pump it directly to the tank. Switch back to flush and pump again. Depending on the length of the hose, this water should be fresh and not have the odor the first pump. This will only need to be done at the beginning of the trip. Regular usage of the head will keep the incoming line with “fresh” sea or lake water. This will be true to a lesser extent with a boat in freshwater system. Forget About It For Boats cannot affect the water in the hull line leading to the head because there is no way to get it there. It will remove the foul sea water odor in the tank. It works as well in saltwater as it does in freshwater systems. The sea salt is just another food source for it. As a matter of fact, sea salt is one of the ingredients in Forget About It For Boats.