Quick Answer: For most boats, the Camplux BW264-N (2.64 GPM, 68,000 BTU) is the best tankless water heater — it fires on 2 D-cell batteries, so it draws nothing from a boat’s limited 12-volt or generator capacity, and it’s built for the outdoor-only mounting a boat actually requires. The catch isn’t GPM, it’s location: Practical Sailor’s reporting on marine tankless heaters found that boater deaths have been attributed to carbon monoxide trapped inside enclosed hulls, and manufacturers including Wolter Water Heater, Paloma, and Rheem-Rudd have issued recalls over exactly that hazard. If the only need is handwashing at a dock with shore power, the electric Stiebel Eltron Mini 3-1 sidesteps combustion entirely, but its roughly 0.82 GPM ceiling rules out a shower.
Every other buying guide on this site assumes a house, where a vent run goes through a roof or a sidewall into open air with room to spare. A boat is a different installation problem: the hull is a sealed, buoyant box, electrical capacity is limited by batteries or a small generator instead of a breaker panel, and the same propane unit that’s perfectly safe bolted to the outside of a barn (see our best tankless water heater for a barn guide) has caused real carbon-monoxide deaths when it ends up mounted inside a cabin instead. This guide covers which models actually work afloat and why where you put it matters more than which one you buy.
The carbon-monoxide risk most boat buyers don’t expect
A tankless propane water heater burns fuel and produces exhaust exactly like any other gas appliance, and Practical Sailor’s reporting is explicit that carbon monoxide is more likely to become trapped on a boat than in a shoreside house because a hull is so much more tightly sealed. That reporting documents real recalls tied to this exact failure mode — Wolter Water Heater, Paloma, and Rheem-Rudd have all had marine tankless models recalled specifically for carbon-monoxide poisoning hazards, and boater deaths have been attributed to the same root cause: a unit installed in or near an enclosed living space with exhaust that can migrate back inside.
This isn’t a reason to avoid tankless water heaters on boats — plenty of boaters run them safely. It’s a reason to treat mounting location as the primary decision, ahead of GPM or BTU. Boat owners who’ve done this successfully mount the unit in a cockpit lazarette, an anchor locker, or another space that vents directly to open air, never inside the cabin, head, or engine compartment, and many add a heat shield and route the flue carefully so exhaust can’t be pulled back in through a nearby hatch. A marine carbon-monoxide detector — Fireboy/Xintex is one established brand — is standard practice on top of that, not a substitute for it.
Our top picks at a glance
| Model | Best for | Fuel / Power | Flow / BTU | Mounting | Price |
|---|---|---|---|---|---|
| Camplux BW264-N | Best overall for most boats | Propane (LP) | 2.64 GPM / 68,000 BTU | Outdoor only — lazarette, locker, exterior mount | ~$240–300 |
| Eccotemp L10 | Best for heavier wash-down demand | Propane (LP) | 3.0 GPM / 75,000 BTU | Outdoor only — lazarette, locker, exterior mount | ~$319–399 |
| Stiebel Eltron Mini 3-1 | Best electric (dock/shore power only) | Electric 120V | Up to 0.82 GPM | Can mount inside — no combustion | ~$219 |
1. Camplux BW264-N — Best Overall for Most Boats
Camplux 2.64 GPM Portable Propane Tankless Water Heater (BW264-N)
- Fires on 2 D-cell batteries with zero wiring or outlet needed — a real advantage on a boat, where a whole-home electric unit's amperage demand doesn't fit a 12-volt system or a small onboard generator.
- CSA safety certification with a flame-failure device and overheat protection, the same model this site recommends for cabins and barns — but on a boat, it must go in a cockpit lazarette or another space that vents to open air, never inside the cabin.
- 2.64 GPM at 68,000 BTU comfortably covers a single transom shower or a deck wash-down hose, the realistic single-fixture demand most boats actually need.
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The BW264-N works on a boat for the same reason it works in a barn: it needs nothing from the vessel except a propane tank and a water line, so there’s no electrical-panel capacity to find. The difference is what “outdoor-rated only” means in this context — on land that’s a weatherproofing spec, but on a boat it’s the carbon-monoxide-safety requirement driving the entire category. Mount it in a lazarette or locker that vents to open air, add a heat shield, and keep a marine CO detector nearby. See our full Camplux lineup for the RV-permanent and ultra-light options if a transom shower isn’t the only fixture you’re trying to supply.
2. Eccotemp L10 — Best for Heavier Wash-Down Demand
Eccotemp L10 Portable Propane Tankless Water Heater
- Same battery-only, zero-electricity setup as the Camplux above, but with more BTU and GPM headroom for rinsing dive gear, washing down the deck, and running a cockpit shower back to back after a full day out.
- 75,000 BTU at 3.0 GPM is enough for one fixture at a time, per Eccotemp's own spec sheet, the same ceiling this site already notes for barn wash racks and off-grid cabins — it isn't built or vented for multiple simultaneous fixtures on a larger vessel.
- Same outdoor-only, vented-to-open-air mounting requirement as the BW264-N — the extra capacity doesn't change where it's safe to put it.
If one transom shower at a time undersells how much hot water the boat actually goes through — a dive charter rinsing gear after every trip, a liveaboard washing dishes and showering in the same afternoon — the L10’s extra GPM over the BW264-N buys some margin without giving up the no-electricity advantage. See our full Eccotemp lineup for indoor-rated models, which exist for houses but are the wrong choice here — indoor-rated means vented through a wall into open air on land, not safe inside a sealed hull.
3. Stiebel Eltron Mini 3-1 — Best Electric, Dock-and-Shore-Power Only
Stiebel Eltron Mini 3-1 Point-of-Use Electric Tankless Water Heater
- No propane, no exhaust, no carbon-monoxide risk at all — the right call for a galley handwashing sink on a boat that reliably has 120-volt shore power or steady inverter capacity at the dock.
- Per Stiebel Eltron's own data, it isn't recommended below a 70°F inlet and tops out around 0.82 GPM at an 80°F inlet — a handwashing appliance, not a shower heater, which matches how little continuous electrical capacity a boat typically has to spare for water heating underway.
- Can mount inside a cabinet or locker since there's no combustion byproduct to vent — the one model on this list where location isn't a safety decision.
Electric only makes sense on a boat when the electrical math actually works, and for most vessels underway it doesn’t — a 240-volt whole-home unit’s amperage draw doesn’t fit a battery bank or a small onboard generator the way it fits a house’s service panel. At the dock with shore power, though, the Mini 3-1 solves the galley-handwashing use case with zero combustion risk. See our full point-of-use water heater guide for the mini-tank alternative if even 0.82 GPM feels thin for how the galley sink actually gets used.
How to choose by what the boat actually has
- Propane tank, a lazarette or locker that vents to open air, no reliable 120V/240V service: the Camplux BW264-N — lowest cost, zero electrical draw, correctly vented by design.
- Same setup, but washing down gear or running a shower after every trip: the Eccotemp L10 — more GPM and BTU on the same battery-only, outdoor-only design.
- Reliable shore power or inverter capacity, galley handwashing only: the Stiebel Eltron Mini 3-1 — no combustion, no venting, no carbon-monoxide exposure.
- No space that vents to open air and no reliable electrical service: don’t install a tankless unit at all until one of those two gaps is solved — this is exactly the scenario the documented recalls and boater deaths trace back to.
- Before you buy anything: confirm where the unit will actually vent, and budget for a marine carbon-monoxide detector (Fireboy/Xintex is one established brand) as a standard part of the install, not an optional extra.
The bottom line
For most boats, the Camplux BW264-N is the best tankless water heater — 2.64 GPM, 68,000 BTU, zero electrical draw, and sized correctly for the outdoor-only mounting a boat requires. Heavier demand steps up to the Eccotemp L10 at 3.0 GPM on the same battery-only design. For galley handwashing at the dock with reliable shore power, the Stiebel Eltron Mini 3-1 avoids combustion entirely. Whichever you choose, the mounting location is the decision that actually matters: Practical Sailor’s reporting on documented carbon-monoxide recalls — Wolter Water Heater, Paloma, and Rheem-Rudd among them — and the boater deaths attributed to the same hazard both trace back to a unit installed too close to an enclosed living space, not to a wrong GPM number.