Marine Water Heater Buying Guide: Tank Size, Heat Exchangers and Safe Install
Quick answer: Pick a marine water heater by crew and use: 3 to 6 gallons for a day boat or weekender, 11 to 12 gallons for a cruising couple, and 20 gallons for larger boats or liveaboards. Choose a model with an engine heat exchanger if you run the engine regularly, so you get hot water underway without shore power or a generator. Match the voltage (120V or 240V) to your AC system, and add a thermostatic mixing valve, because engine-heated water can be scalding.
Hot water turns a boat from a day trip into a place you can actually live on: a warm shower after a swim, hot water for dishes, and a clean rinse at the end of the weekend. Marine water heaters are built to handle motion, vibration and salt air, and most can heat from two sources. This guide covers tank size, heating sources, voltage, tank material, installation and safety.
Step 1: Choose the tank size
| Use | Tank size | Notes |
|---|---|---|
| Small boat, occasional rinse | 3 gallons | Fits tight spaces, recovers quickly |
| Day boat or weekender | 6 gallons | Two short showers or dishes for a small crew |
| Cruising couple | 11 to 12 gallons | The most common size on 30 to 40 ft boats |
| Larger yacht or liveaboard | 20 gallons | Multiple showers back to back |
Usable hot water is more than the tank size, because hot water is mixed with cold at the tap. A thermostatic mixing valve stretches the supply further by tempering very hot water down to a safe temperature.
Step 2: Decide on heating sources
- AC electric element: heats water on shore power or generator. Most marine units use an element around 1,500 watts, which is a meaningful load on a 30 amp shore power service, so plan your other AC loads.
- Engine heat exchanger: a coil inside the tank plumbed into the engine's closed coolant loop. Running the engine heats the water for free. Heat exchanger position (front, rear or side) matters only for how the hoses reach your engine, so pick the one that suits your layout.
- Element only: fine for boats that live on shore power or rarely run long passages.
Engine-heated water can approach coolant temperature, which is hot enough to scald. Fit a mixing valve on the hot outlet whenever you use a heat exchanger.
Step 3: Match voltage and electrical setup
North American boats almost always use 120V heaters. 240V versions suit boats wired for 240V or European shore power. A 12V low-wattage heater exists for small boats, but it heats slowly and draws a lot from the battery; it is best used to keep water warm while charging or underway. Put the heater on its own correctly sized AC breaker, and never switch the element on with an empty tank, because a dry element burns out in seconds. Our shore power guide covers AC capacity.
Step 4: Pick the tank material
- Stainless steel: the longest-lasting choice and the best in salt air. Highest price.
- Galvanized steel: lower cost, good service life if kept dry and drained when laid up.
- Epoxy-coated (white): budget friendly and fine for lighter use.
Some tanks use a sacrificial magnesium anode; check it each season and replace it when it is significantly wasted.
Step 5: Install it right
- Secure mounting: a full tank is heavy. Bolt it to a structural surface that can handle the weight in a seaway.
- Pressure relief valve: every heater needs a working temperature and pressure relief valve. Pipe its discharge somewhere safe and visible.
- Check valve on the cold inlet prevents hot water from backflowing into the cold side.
- Coolant hoses: keep runs as short as practical, use rated heater hose and double clamps, and bleed air from the engine cooling system after installation.
- Bypass valves make winterizing easy so you are not filling the tank with antifreeze.
Our marine hose and clamp guide covers hose types and proper double clamping.
Common water heater mistakes
- Powering the element with the tank empty, which destroys it instantly.
- Skipping the mixing valve on an engine-heated system.
- Buying too small for the number of people showering.
- Weak mounting that lets a heavy tank shift in rough water.
- Not draining or bypassing it for winter in freezing climates.
Our picks for marine water heaters
All in stock at Boat Supplies Direct.
Compact heaters for small boats
Whale Seaward 3 Gallon with Heat Exchanger or the Whale 3 Gallon 12V for boats without AC power.
6 gallon heaters for weekenders
Whale Seaward 6 Gallon with Rear Heat Exchanger, the stainless Seaward 6 Gallon with Front Heat Exchanger, or the Kuuma 6 Gallon 120V.
11 to 12 gallon heaters for cruisers
Whale Seaward 11 Gallon with Rear Heat Exchanger or the Raritan 12 Gallon with Heat Exchanger.
20 gallon heaters for larger boats
Raritan 20 Gallon with Heat Exchanger for liveaboards and boats with multiple heads.
Safety and maintenance parts
Whale Seaward Thermostatic Mixer Valve and the Whale Seaward Magnesium Anode.
Shop all marine water heaters →
Buying parts often? Get Dock Pass.
Free shipping on every order, 5% off eligible products and 60-day returns for $49 a year. See how Dock Pass works.
Frequently Asked Questions
What size water heater do I need for my boat?
About 6 gallons for a day boat or weekender, 11 to 12 gallons for a cruising couple, and 20 gallons for larger boats or liveaboard use.
How does a marine water heater heat water while underway?
A heat exchanger coil in the tank is plumbed into the engine's closed coolant loop, so hot coolant heats the water whenever the engine runs.
Do I need a mixing valve on a boat water heater?
Yes if you use an engine heat exchanger. Engine-heated water can get hot enough to scald, and a thermostatic mixing valve tempers it to a safe temperature.
Can I run a boat water heater on 12 volts?
Small 12V heaters exist, but they heat slowly and draw heavily from the battery. Most boats heat water with AC power or the engine.
How much power does a marine water heater use?
Most AC models use an element around 1,500 watts, roughly 12 to 13 amps at 120V. Check the label on your model and plan your shore power loads.
Why did my water heater element burn out?
The most common cause is switching it on when the tank was empty or had an air pocket. Always fill and bleed the tank before powering the element.
This guide is general information. Follow your heater and engine manufacturer's instructions, ABYC electrical and plumbing standards and local codes, and use a qualified marine technician for AC wiring and coolant loop installation.
