What is a common sign of battery failure on a boat?

Difficulty starting the engine is a common sign of battery failure on a boat because the battery is responsible for providing the power needed to crank the engine. When a battery is weak, failing, or depleted, it may not supply sufficient voltage or amperage, leading to slow cranking or complete failure to start. This issue can manifest as clicking sounds when the ignition is attempted or a lack of response altogether.

The other options, while indicative of various issues that may arise on a boat, do not directly signify a problem with the battery specifically. Inconsistent fuel readings may relate to fuel system issues or a faulty gauge, excessive water in the bilge could indicate a leak or pump failure, and vibrations in the hull might suggest engine or propeller misalignment rather than a battery concern. Therefore, identifying difficulty starting the engine primarily points to potential battery failure.