The boat sat on its lift for three weeks in August with the tank half full. First start back, the F115 idles a bit lumpy and the gauge throws a water in fuel warning before you've even untied. That timing isn't a coincidence. A half-empty tank on a hot lift breathes humid air every night, and with ethanol blended gas, that moisture has somewhere to go.
What the switch actually sees
The warning comes from a water detection switch in the filter housing on the engine. Water is heavier than gasoline, so it settles to the bottom of that housing, and once enough collects to reach the switch, it closes. The ECU logs it as a water in fuel filter entry, detection switch on. All Yamaha families have this in some form, from the small F20 up to the XTO, and the HPDI Z series reports the water detection switch state in its live data.
The switch knows about that one cup of fuel and nothing upstream. It can't tell you how much water is in the tank, whether the boat-side water separating filter is already full, or whether the fuel at the dock was bad.
At the dock, before you go anywhere
- Shut down. Running on with water in the filter risks pushing it toward the injectors.
- Pull the cowling and look at the engine filter cup. On many engines it's clear enough to see a layer. Drain or replace per the manual.
- Do the same for the boat-mounted water separating filter if there is one. It usually holds more water than the engine filter, and if it's full, the engine filter is the overflow.
- Draw a sample from the separator drain or the filter into a clear jar. Two layers, or cloudy fuel that doesn't settle, tells you the tank has a problem the filters are only catching.
- Prime the system and start. Let it idle while you watch whether the warning returns.
When the filter is dry and the warning stays
Now the switch circuit becomes the suspect. The check steps the app lists for this entry go in a sensible order: check for water, check the engine monitor, check wiring and connector, then check the switch for continuity. With the M-JET DIAGNOSTIC connected you can watch the state of the switch live while you wiggle the harness at the filter housing. A state that flips with a light touch on the connector is a wiring problem, and salt crust on those pins is common on boats kept in the water.
A switch that reads "on" with a clean, dry cup and a sound connector is a failed switch. That's a cheap fix compared to chasing fuel that was never wet.
Where the water comes from
Most of what turns up in these filters isn't from the fuel dock. It's condensation in the tank, rain finding its way past a worn fill cap O-ring, or water forced in through a vent fitting mounted low on the hull side. Ethanol fuel absorbs a certain amount of moisture and carries it through. Past that point it separates, and the ethanol takes some of the water with it into a layer at the bottom. That layer is what the engine picks up when the pickup tube is near the bottom and the boat pitches. Keeping the tank fuller during idle weeks and checking the fill cap seal do more than any additive.
How serious is a single warning
A single entry after a long layup, with a cup that showed a few drops, is common and usually ends at the drain plug. Repeated entries every few trips mean the tank is still feeding water, and filters alone won't fix that. If the engine ran rough with water present, read the rest of the fault memory too. Injector or misfire-related entries after a water event are worth a closer look before a long run. The water in fuel filter page covers the switch, the filters and the follow-up checks in more detail.
