Get under the sink and follow the hose, it should lead straight to a below the waterline thru-hull. The water level inside the hose stays at the waterline. Heavy solids will fall through and liquids will be replaced by those entering the drain. This system does not work very well when the boat is docked as floating solids will remain in the hose and over extended periods can clog it. On the other hand when the boat is moving and water is rushing across the opening it creates a syphon which pulls the water out of the hose. The drain hose is also prone to gargle as the boat falls and rises among the waves altering the waterline level. On some boats as they heel the sink may get filled with water. I like to keep the sink drain thru-hull open to keep the water in it from getting stagnant but it is important to inspect the hose and clamps frequently to prevent any accidents.