Water in fuel tank.

[tsubaki]

And there are any number of scenarios that can play out, depending on your situation.
You could just pump it out through the fuel line with the fuel pump.
Add another 5 gallons of fresh fuel and pump it out again but monitor this fuel for excessive water/trash say with a clear container.
Install a water separator/filter and let it go at that. Changing the filter after using one tank.
All along checking for possible fuel leaks/smells in the bilge and other locations.