A boat’s freshwater tank is the quiet problem. It sits part-full for weeks, tucked under a berth or hard against the engine bay where it warms nicely, feeding a shower that gets used once a fortnight. That is close to a textbook description of where Legionella likes to breed: warm, still, low-turnover water with a fine spray at the end of it.

Most narrowboat and cruiser owners never give it a thought, and on a purely private craft the risk is mostly your own to manage. The moment a boat is hired out, used for paid trips, or moored on a marina that hands water to the public, the same plumbing stops being a private matter and becomes a duty to other people.

This guide takes the control measures you would use in a building and translates them to the realities of small systems afloat.

Why a boat is good habitat for the bacteria

Legionnaires’ disease is caught by breathing in small airborne droplets of contaminated water, not by drinking it [5]. So the parts of a boat that make an aerosol — the shower above all, but also a spray tap or a deck wash — are the parts that matter most for exposure [3]. The water can sit in the tank for a month; it only becomes a problem when it leaves an outlet as a fine mist near someone’s face.

Several things conspire afloat. The freshwater tank is usually small and rarely run down and refilled, so the same water lingers, and it is often warm — engine-bay heat on one side, sun on a steel hull or a dark deck on the other — drifting up into the 20–45°C band where the bacteria multiply fastest. The calorifier (the boat’s hot-water cylinder) may be heated off the engine and only reach a proper temperature on travelling days, spending the rest of its life lukewarm. Flexible hose, plastic tanks and dosing points all carry biofilm, the slime layer that shelters Legionella. And boats get laid up for whole seasons with water left standing in the system.

None of that is exotic. It is the ordinary condition of a boat that is loved but used intermittently.

Where the duty actually bites

There is no separate “boat Legionella law”. The control duty sits in the same place it does ashore: with employers and with people in control of premises in connection with a work activity, applied through a risk assessment and a written scheme of control [4]. What changes is whether your boat counts as a work activity.

A private craft you use yourself is, in legal terms, largely your own affair. The sensible owner still keeps the system clean and the water moving — the checklist below is written for you too — but you are managing personal risk, not discharging a statutory duty.

A hire or charter boat, or a trip boat carrying paying passengers, is a business. You are putting members of the public in front of a shower or a tap you control, so the duty to assess and control the foreseeable risk applies, and you should expect to hold a suitable risk assessment, a scheme of control, and records that show it is being followed [1]. The closest regulated parallels already exist in this corpus: hire fleets sit somewhere between Temporary events and festival sites: portable water systems, where small systems are stood up and torn down for short, intense use, and Legionella control in caravan parks and holiday parks, where the operator is responsible for water handed to a stream of short-stay guests. Borrow the thinking from both.

A marina is in control of shared infrastructure: water points along the pontoons, communal shower and ablution blocks, sometimes a pressurised ring main feeding dozens of berths. That is squarely a premises-in-control situation, and the ablution-block showers are the highest-risk part because they are aerosol-generating and often lightly used out of season [3]. A marina cannot reasonably police what happens inside each owner’s boat, but it is responsible for the water up to and including the points it provides.

A practical checklist for water afloat

Treat this as four short routines rather than one big job. Tick the items and date them — a hire operator or marina will need that record; a private owner will simply find it useful.

Recommissioning at the start of the season

  • Drain the freshwater tank, the calorifier and the pipework down completely.
  • Open the tank, clean out any sediment, slime or debris, and wipe down the internal surfaces you can reach.
  • Disinfect the whole system: dose with a chlorine-based sanitiser approved for potable water at the concentration the product specifies for the tank volume, fill so every outlet draws treated water, then leave it to stand for the stated contact time.
  • Flush thoroughly — run every tap and the shower until the chlorine smell clears — and refill with fresh water.
  • Bring the calorifier up to a proper hot temperature before first use; if it is engine-only heated, give it a run or use shore power so the stored water actually gets hot rather than warm.

Routine use through the season

  • Run every outlet, including the shower and any rarely-used tap, at least weekly; a tap that never flows is the one that grows a problem.
  • Run the shower for a minute or two before stepping under it after any quiet spell, ideally with the head removed or pointed low and the window open so you are not standing in the first aerosol.
  • Keep the cold side genuinely cold. Stowed where it warms, the tank drifts into the growth band, which is the same battle covered in Cold water temperature guidelines to prevent Legionella and, for the tank specifically, Cold water storage tanks: keeping temperatures low.
  • Turn the tank over. Don’t top up a half-full tank for months; run it down and refill so the water is fresh.
  • Keep the tank lid and inspection hatch sealed against muck, insects and warmth.

Laying up for winter

  • Drain the system down fully — tank, calorifier, pipe runs, the shower hose and any low points where water pools.
  • Leave drain valves and taps open so nothing sits standing over the lay-up.
  • Plan to recommission (the routine above) before the boat carries anyone next season, not after.

Shore-supply hose hygiene

  • Drain the filling hose after every use; a hose left coiled full of water in the sun is a small, warm, stagnant Legionella reservoir feeding straight into your tank.
  • Store it capped and off the ground, not lying on a warm pontoon.
  • Keep the hose for potable water only, label it, and don’t share it between the bilge-rinse job and the drinking-water fill.

On the temperatures

The targets afloat are the same ones used ashore, even if the kit makes them harder to hold: cold water kept below about 20°C, hot water stored at around 60°C and reaching the outlet near 50°C within a minute or so [2]. The honest difficulty is that an engine-heated calorifier won’t sit at 60°C on a non-travelling day, and a tank in the sun won’t always stay below 20°C. You close that gap by keeping the water moving and turning it over, not by pretending the numbers are met when they aren’t.

This is general guidance, not a specification for your vessel. A 28-foot private cruiser, a hire narrowboat fleet and a marina with communal showers face genuinely different risks, and only a competent person working from an assessment of your actual system can set the right measures, temperatures and check frequencies for it. Nothing here is legal, medical or marine-engineering advice.

FAQ

Is the water in a narrowboat tank safe to drink?

Drinking it is not how Legionnaires’ disease is caught — the route is inhaling aerosol, mainly from showers [5]. That said, a tank that has stood warm and part-full for weeks can harbour more than Legionella, so many boaters treat the tank supply as washing water and carry separate drinking water. If you do drink it, a clean, regularly turned-over tank and a potable-grade filling hose matter most.

Do I need a Legionella risk assessment for my private boat?

If it is genuinely private and you use it yourself, there is no statutory duty driving a formal assessment — though the cleaning and flushing routine here is still worth doing for your own sake. If you ever hire it out, run paid trips, or take fare-paying passengers, you are running a work activity and should hold a suitable risk assessment and scheme of control [1][4].

How often should I sanitise a boat water tank?

At minimum, as part of recommissioning after any winter lay-up or long period out of use, and again if the water develops a taste, smell or discolouration, or after any work that opens the system. A boat in constant liveaboard use with water turning over still benefits from a clean and disinfection at least annually. Your risk assessment, where you need one, sets the firm frequency.

What to do next

Before the next person showers aboard, do the cheapest high-value job: pick the least-used outlet — usually the shower or a spare basin tap — and run it hard for a couple of minutes, then note the date you did it. If you operate a hire fleet or a marina, turn that single action into a standing weekly routine with a dated record per boat or per ablution block, because an outlet you can prove was flushed is the difference between control and hope. A simple recurring log keeps those small, easily-forgotten tasks visible until they become habit.

Sources

[1] HSE, “Legionnaires’ disease - what you must do”. https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm [2] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [3] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [4] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [5] NHS, “Legionnaires’ disease”. https://www.nhs.uk/conditions/legionnaires-disease/