My boats water smells like rotten eggs and sulfur, how to fix it

Whoah! That unmistakeable smell of rotten eggs in your boats water tank is one you don’t soon forget. But how do you rid your boat of this nasty odor?

If you own a boat with a fresh water tank and/or hot water heater, you know what I mean. Its a very distinct odor that is not pleasant by any means. It will smell either like rotten eggs or sulfur but why?

There are several reasons why your boat has this smell. And the solutions to the smells are easy to put in place.


  1. There is algae, mold, or bacteria somewhere in the system
  2. Age
  3. An anode somewhere in the system went bad

First, lets talk about how this smell comes about in your boats fresh water system. When algae or mold is the underlying cause the smell can be anywhere in the system. Including the tank, hoses, pumps, or other accessories. This occurs from standing water in the system. If a boat sits for a while without use the water will start to form algae within the system.

The smell can also form over a long winter, even if you winterized your fresh water system. If you remember from above, heat creates a breeding ground for mold. The same holds true for your fresh water tank.

Tackling algae, mold or bacteria in your fresh water system

If you feel the root cause for the rotten egg smell in your boats water system is due to mold, then we need to flush and cleanse the entire system. Whats the best way to rid your boat of sulfur and bacteria you ask? Why bleach of course. You need to be smart when using bleach, it can deteriorate the tank, fittings and lines if used improperly.

Flush the fresh water system

How do you safely use bleach to sanitize your boats fresh water system? Before adding bleach to the boats water tank you need to empty the fresh water tank including the hot water heater. Open all faucets, and showers. Turn some faucets to hot water to drain the hot water heater (thats something people forget to do). The hot water heater by nature is a breeding ground for bacteria.

Make your bleach solution

Now that you’ve fully emptied the boat you need to fill it up with a bleach solution. Depending on how intense the smell is you can create your own ratio. Some people like a ratio of one cup of bleach for every 10 gallons of water. First, find out how big your fresh water tank is so you can add the appropriate amount of bleach to purge the system. Remember more bleach is not better. After filling up the water tank with your bleach solution, run all faucets, showers, etc until they smell like bleach. The premise behind this is so you know you’re running bleach throughout the entire water system. Be sure to fill up the hot tank as well and run a faucet with hot water till it has a bleach scent too.

Let it sit for 30-60 minutes to marinate in the boats water system and start to kill the bacteria. If you want, take the boat for a ride and let the water slosh around. After that flush the entire boat like you did prior. Fill the boat up with water and flush the tank until you smell crisp, clean water that has no bleach odor to it.

Rid the fresh water tank of bleach

Once you’ve eliminated the bleach smell from the lines and the water no longer smells like sulfur or rotten eggs you have fixed the issue. Its now safe to fill your boat back up with water for personal use. If you have a water filter built into the boat I would replace the charcoal based filter afterwards too.

I would flush the tank once with a clean tank before going forward with personal consumption. If the problem persists, its time to move onto the next possible reason why the fresh water smells.

Layout to tracking down hoses on a boats freshwater system


Like anything boating related, age leads to replacement. If the water is cloudy or there is debris coming out of the faucets the hoses have gone bad or the tank is dirty. To test whether its the hoses deteriorating or the tank you can try a few different tricks.

  1. Physically inspect the lines. If the hoses are white and look discolored or cloudy then its time to replace them.
  2. Turn the faucets on. If only one sink is emitting debris or cloudy water chances are its only that hose. Not the tank or multiple lines.
  3. Is your tank plastic or metal? If its plastic and you can look at it you’ll be able to see if theres sediment floating in the tank. If you have a metal tank, do you have an inspection plate? Pop that open and see if you have debris in the tank.

Solutions to bad hoses

If it turns out the hoses have gone bad, its straight forward, replace the hoses that went bad. Go through all the lines and make sure to replace any questionable ones while you have your wrenches out. Simple, right?

Solutions to a dirty tank

You’ve inspected all hoses and they look good. This means you need to clean the water tank. If you can visually inspect the tank because its plastic do so. If you have an inspection plate pull that off and check inside the tank. But be sure to drain the water out of the boat first this way you can actually see whats going on.

You see theres dirt and sediment in the tank. Through the inspection hatch place several ice cubes in the water tank. Fill up the tank with enough water to help slosh the cubes around. A couple inches will suffice. Too much water will lift them off of the bottom of the tank and do you no good.

Now take the boat out for a ride and let the ice cubes scour the tank. The goal here is to loosen up all the debris. After you feel confident you’ve loosened up all the sediment, head back to the dock. Fill the tank up and flush it once or twice, until all the debris is gone and the water is clean again. You can compare this method to using a Brillo pad on a pot that you burnt food in.

Afterwards, I would perform the bleach treatment mentioned above to sanitize your tank.

Although this is not one of the main causes of rotten egg smell in the fresh water system, its not good for the boat either. You should address this issue while going through the system.

An anode went bad in the system

The the last step in your pursuit of a clean smelling fresh water system on your boat is to check the anode rod. If you’re thinking to yourself “what the F@$% is an anode rod”, ill explain. I was once perplexed like you. This is a controversial solution. Some say this is not the cause of sulfur smell on board your boat and to bleach the water tank instead. Im including this in my discussion to arm you with every possible option.

What is an anode?

Anode rods are in your boats hot water heater. They are long cylinders composed of aluminum or magnesium, attached to a steel wire core. The reason they make them out of aluminum or magnesium is because all metals rust. But these two will rust away completely before causing any corrosion in the tank itself.

If 6″ or more of the core is exposed by corrosion its time to replace it. Doing so will increase the lifespan of your hot water heater. Soft water leads to decaying them quicker. Keep this in mind, if using a soft water filter to fill your boats water tank.

how to check to see if the anode rod on your boat is good or bad in your boats hot water heater
New vs. Old Anode Rod

Checking your hot water heaters anode rod

Enough science talk, how to know if this is the root cause. The first way to check is if only the hot water on your boat is foul smelling. If thats the case chances are the anode has gone bad and needs to be replaced. To be sure unscrew the anode from the hot water heater with a ratchet and inspect it. If it looks worn away, replace it. If not flush your hot water tank with some bleach to clean and sanitize the tank.

Some plumbers will recommend switching from magnesium anode to an aluminum one to fix the issue. If it turns out the rod is too far gone then you’ll have to replace the hot water heater, it may have caused harm to the tank.

As you can see, its not the simplest fix but also not catastrophic if you need to replace the rod. Remember, the easiest way to narrow it down to the anode is if only the hot water smells like sulfur or rotten eggs.

Prevention & Maintenance

Once you resolve the rotten eggs dilemma the best course of action is use. Like anything on a boat the best thing you can do for your boats water tank is use it, go through water, keep it flowing. This prevents deterioration, debris and mold. Another good prevention tip is to put water treatment in the tank to keep the water from going bad. Comparable to how you treat the holding tank on your boat.

If I have a customer not using their boat for a little I splash a capful of bleach into the tank. This is proactive in preventing rotten egg smell in the boat. Or if the boat  is going to the yard to have work done, ill splash some bleach in the water tank.

These are the top three sources for bad smelling water within your boats water tank. None of them are difficult to resolve. Like anything else with a boat it takes time and patience to diagnose the issue.

As always if you have any questions feel free to drop a line below. If you have any other questions about mold and mildew on your boat be sure to read my article on preventing mold and mildew. Another good source for smells would be my article on diesel smell onboard your boat.

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