"Water"...... that really says a lot. Thanks for the insight!!
IF you knew very much about fuel tanks, you wouldn't need another word. But, I'll offer some education.
Scenario: Fuel tank filled to 20% capacity as an example. That means the fuel tank contains 80% air. Air contains a varying degree of moisture. The fuel tank is not sealed; there is a vent, otherwise it would vacuum-lock as you burned fuel. Gasoline has a high coefficient of expansion (due to temperature). During the day, especially a hot one, the gasoline expands, and air is expelled from the tank. At night, it contracts, drawing in cooler air (that's often moisture-laden) via the vent.
The moisture condenses on the sides of the tank, then runs down the side as droplets. Water is heavier, so it goes to the bottom of the tank, where it is trapped under the layer of gasoline above. Once there, it can never "evaporate back out." This cycle continues over an over, with any added moisture accumulating as a trapped substance. Given conditions bad enough, you could theoretically end up with more water than gasoline over time! Some people store a boat for long periods of time with the tank mostly empty, setting up this disaster. When the boat is bouncing around, the water and fuel become combined. When more calm, more water settles and the fuel system pickup sucks up more of this water that has settled. Water doesn't burn very well, just in case you weren't aware.
The issue is best avoided by filling the tank to FULL when a long storage period is expected. The gas should be properly treated with a fuel preservation additive to avoid other storage issues, but the simple act of filling the tank will avoid the water issue.
The OP should pull a quart of fuel via the existing fuel line and put it into a glass or suitable plastic bottle to let it settle. If water is present, it will become obvious by visual observation after a few hours.