Below is a typical sailboat exhaust installation. The waterlock silencer (muffler) is the collection point for water in the exhaust system when you shut the engine down, and as such needs to be the lowest part of the exhaust system. You need a drop of about 9” from the exit of the exhaust elbow (X on the drawing below) to the inlet of the waterlock muffler. If you are unable to accomplish this then we suggest using a high rise exhaust elbow which will raise point x up by the required amount.
If you are using a plastic waterlock muffler or a fiberglass one, then we suggest a minimum of 12” length of hose between the exit of the exhaust elbow and the entry point of the waterlift muffler. This allows enough time for the water in the exhaust system to cool the gases down so you do not have any muffler problems.