Piston engines don’t run well on water or anything other than what they’re designed to burn. Water can be present in the fuel from condensation, particularly if the tanks are not kept full. Since water is heavier than fuel, it collects at the lowest points in the tanks, where the fuel strainers are located. That’s why we sump the fuel before starting up the airplane. If your airplane runs on 100LL, you’re most likely also used to looking for that slight blue color in the fuel tester to ensure that the correct type of fuel is in the tanks. Any water in the fuel tester will appear as a clear bubble below the blue fuel.
None of this should be news to you. But in some airplanes it may not be enough to simply push a fuel tester into a strainer to remove the water from the fuel. Airplanes, particularly those without much dihedral such as the Cessna Cardinal, may collect water in the outboard portions of the wings or other sections where there are no strainers.
