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  • Writer's pictureAndrew Lambert

Three Waters Below Deck: Knowing The Difference For Your Boat

Updated: Nov 8, 2023

Three Types of Water In Marine Plumbing


Your boat has three types of water in its plumbing system: fresh, gray, and black. It is very useful to know where your water is coming from, how your water is being discharged, and how to safely store fresh water.


You may wonder what kind of water goes where and why it matters. We all know boats are not connected to a water source like a sewer or well, so where does your fresh water come from?


This article will discuss the 3 Types of water on your boat as well as Marine Plumbing Maintenance.


Water In & Water Out


Fresh Water


The Mobile Mariner | Jensen Beach | Florida | Marine Plumbing Water

When it comes to marine plumbing, you should know your waters. Your fresh water is the clean water you use onboard. It may be sourced from a public water source, bought from a store, or collected (rain or raw) and purified by a watermaker through reverse osmosis. This water is stored in a 10-100 gallon freshwater tank that is stored safely on the boat. To use your freshwater, you will need a 12-volt freshwater pump to draw the water from your tank and deliver water to the faucets as needed. This pump pressurizes the water to create an on-demand water supply for you, and you can purchase an electric or manual pump.

Out of the three types of water, your fresh water is what you will use to cook with, shower with, and wash your hands with. Fresh water can be used up fast, so remember to use your water wisely. While it can be boiled for tea or coffee, it is recommended to have fresh bottled water for consumption purposes.


Fun Fact: The average adult consumes between 91-125 liters a day.



The Mobile Mariner | Jensen Beach | Florida | Marine Plumbing Water

Gray Water


Gray water is the used water on your boat.


While freshwater has not been riddled with any pathogens, gray water is a type of wastewater. This water comes from your shower, galley, sinks, laundry, and bath. Gray water from your sink will flow down a drain and then through a P-trap, exiting out into the water. Gray water from your shower, however, uses a shower sump pump to drain the water. If your boat does not have a pressurized water system, you can pour boiled water into the sump.



The Mobile Mariner | Jensen Beach | Florida | Marine Plumbing Water

Black Water


Black water is your sewage water, and it comes from your toilet. When you flush, your black water will pass through a y-valve and travel through a hose, making its way back to the holding tank. Rather than draining overboard, your black water must be stored in a holding tank until your boat can be properly pumped and disposed of. There are 546 pump-out locations located in Florida alone.


Three miles away from land, it is legal in the US to dump your black water overboard. If you are pumping overboard, you will need to switch the Y-valve to the overboard position, open the head overboard thru-hull, and use the head. If you have a gravity pump, physics will take over when you open the tank outlet thru-hull.


Marine Plumbing Maintenance


Now that you know your main three boat waters, it is important to understand the maintenance that goes with it. For most, a boat is a luxury item, but it should be treated like that by all and be given TLC. While some will leave their boat at a full-service marina, there are important things that all boat owners should keep in mind when it comes to their marine plumbing.


For example, your fresh water tank should be an investment. Stainless steel and fiberglass tanks are ideal materials (plastic can be used in place if you are looking for a budget-friendly option).


In marine plumbing, it is important to make sure everything is working right. Your freshwater system is no exception. It will need annual maintenance to keep it up to par and prevent contamination. This involves draining your system completely, filling it with a water sterilizer, and then draining it again. This takes place over a couple of days. An in-line filter should be added to your freshwater system to ensure your water is clean and drinkable (although it is recommended to have bottled water for drinking on board).


Electricity also matters when it comes to your marine plumbing systems. If your electricity supply is low, it can affect how well your pumps work. Your freshwater pump, macerator pump, and shower sump pump all run off electricity.


While there is an option to have a manual pump (there are even manual macerator pumps), some will still go with an electric pump for convenience.


Most boat owners can manage their own plumbing because there aren't too many complicated pipes, and they are not behind the walls. However, annual service should be done by a marine mechanic to ensure that everything is a-okay with your marine plumbing. A professional will check for any signs of corrosion or damage and make sure that all connections are secure and free of leaks. They will also ensure that your fresh water supply isn't contaminated and that all your pumps are working properly, too.


When you work with The Mobile Mariner, you are choosing quality work from a dedicated team. We understand that it takes a thorough inspection of a marine plumbing system to see every aspect of it. We will inspect and make any necessary marine plumbing repairs or replacements as needed.











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