How Water Gets Into Boat Fuel (and How to Stop It)

BoatUS describes the classic check: pump or drain a pint or so of fuel from the boat side of the system (or empty the fuel filter) into a clean, clear glass jar and let it sit a few minutes. Boating Magazine adds what to look for:

• Clear amber fuel, nothing at the bottom: no free water in this sample.
• A clear bubble or layer at the bottom: water (gas floats on water).
• A cloudy or whitish, gel-like layer: phase-separated ethanol fuel. Boating Magazine says this fuel can't be burned and may need professional disposal.
• Specks of dirt or rust: sediment from the tank or bad fuel.

Do it outdoors, away from ignition sources, and pour the sample into an approved container afterward.

Look in the clear bowl of your fuel/water separator, and on outboards, the small see-through filter on the engine. Yamaha notes its on-engine filter may have a red ring inside: if it's floating, there's water in the trap. Don Casey's rule of thumb for diesels: if you have to drain water from the primary filter bowl more often than about every 100 engine hours, the tank needs attention. Water every time means an active leak; a little once in a long while points to fuel quality or condensation.

If the water showed up right after you bought fuel, the dock's fuel may be the culprit. Don Casey suggests jar-testing doubtful fuel before it goes in your tank and buying elsewhere if you see water or dirt. Keep your receipt and tell the marina. Buy where boats fuel often so the fuel turns over.

Unscrew the deck fill cap and look at the O-ring or gasket. BoatUS says most fills rely on a simple rubber O-ring that dries out, cracks and lets water through. Don Casey adds that some O-rings are simply missing. Replace it if it's flattened, cracked, hard, or loose in its groove. It's a cheap part; bring the cap or the old ring to the marine store to match it. Make sure the cap threads down fully and isn't cross-threaded.

Pour a cup of water around the closed fill on a calm day and watch. If it puddles around the cap instead of draining away, rain, spray and washdown water sit on your O-ring all the time. BoatUS notes that a fill should never be recessed so water collects around it, and that some makers sell spacers (thick washers between the fill flange and deck) to lift the cap above the puddle. Don Casey points out that side-deck fills on sailboats can spend a lot of time underwater when heeled. Raising or moving a fill means re-bedding it; see our deck hardware re-bedding guide.

Find the vent fitting, usually on the hull side or transom. BoatUS and Don Casey both warn that a hull-side vent can be pushed underwater by waves, heavy rolling or deep heeling, and a transom vent by a following sea. Check:

• Is the vent low enough to get dunked in normal use?
• Does the vent hose have a loop or rise to trap splashes before they reach the tank? BoatUS recommends one.
• Is the screen intact and the vent facing aft?

An aft-facing clamshell cover over the vent can help, according to BoatUS. Newer EPA-compliant systems may vent through a carbon canister instead; see our fuel fill and vent hose guide before changing anything on those.

Boating Magazine adds a few vent points worth checking:

• No sags in the vent hose between the tank and the fitting. A low spot traps fuel (or water), blocks venting and can make the pump nozzle keep shutting off at the dock. The hose should run across the top of the tank and then rise to the fitting so anything inside drains back.
• Point the hose aft when you wash the boat. Spraying forward along the hull side shoots water straight into the vent opening.
• Some replacement vents have a built-in splash guard or water trap. If you change one, use a vent the maker lists as Coast Guard compliant, and mount it high and angled aft and slightly down.

Open the deck plate or hatch over the tank. BoatUS calls the sending unit (the fuel gauge sensor on top of the tank) a common water entry point because its gasket and screws can let in water that pools on the tank top. Look for:

• Standing water, rust stains or salt crust on the tank top
• A hard, cracked or squeezed-out sending unit gasket, or loose screws
• Leaking deck plates or hatch seals right above it

If you remove the sender, BoatUS suggests marking its position first, since most have a barely noticeable irregular screw pattern. Our fuel gauge sender guide covers the job.

On a poly (plastic) tank, look across the top. BoatUS warns that these tanks can sag into a concave shape over time, holding a puddle and keeping the sender from sealing. Mild cases can be helped by keeping the area dry; severe cases may mean a new tank.

A lot of water arrives by hose:

• Don't aim a hose or pressure washer at the fuel fill, vent or sending unit cover.
• Keep the fill cap on tight and dry it before you open it to refuel.
• Portable tanks: BoatUS suggests taking them off the boat when not in use and storing them somewhere dry and ventilated, or covering them so rain can't reach the cap and vent screw. Close the vent when stored.
• Cover the boat, or at least the cockpit, so rain doesn't pool around the fill and tank hatch.

If water keeps appearing and every seal checks out, the tank may be the entry point. BoatUS explains that aluminum tanks set in foam can trap moisture against the metal and corrode through, especially in salt water. Warning signs are a fuel smell in the bilge, fuel in the bilge, or water in the fuel after rain or washing. A technician can drain and pressure-test the tank. BoatUS notes aluminum tanks usually aren't repairable once they leak. See our fuel tank inspection and replacement guides.

Once the entry point is fixed, deal with what's inside:

• Small amounts in a gasoline system: drain the separator bowl and replace the filter element; check the on-engine filter.
• Larger amounts or phase separation: the tank needs pumping out from the very bottom. Don Casey notes the pickup tube sits a bit above the bottom, so you can't get it all through the engine line. A marine shop or fuel polishing service can remove it.
• Diesels: polishing (filtering the whole tank) removes water and sediment. See our diesel fuel polishing guide.

Don't assume the fuel above a phase-separated layer is fine, either. Mercury explains that when the water and ethanol drop out, the gasoline left on top has much less ethanol, which can lower its octane by as much as 3 numbers and cause engine problems. Ask the shop whether the whole tank should be pumped out rather than just the bottom layer.

Additives won't rescue water-logged fuel. Yamaha says no additive will remove water or restore fuel that's gone bad, and Boating Magazine says stabilizers prevent fuel going bad but don't make bad fuel usable.

After repairs, recheck the separator bowl after a rainstorm and after your next few washdowns. Log what you found, the parts you replaced and the date in BoatMend, and add a reminder to inspect the fill O-ring every season.