Fuel systems are usually fed by gravity, at least until the fuel leaves the tank, where sometimes (always if the tank is lower than the engine) a pump pushes it toward the engine. This means that fuel is generally extracted from the bottom of the tank (or slightly above the very bottom to avoid having small particles from the tank clogging up the fuel.
Avgas is weighs 0.72 kg/l and Jet fuel 0.82 kg/l, water weighs 1.0 kg/l this means that aviation fuel is less dense than water, meaning it will float on top of the water.
Or if you want to think of it the other way around, water is heavier and will sink to the bottom, right where you're trying to get your fuel to feed to the engine.
As we all know, water doesn't burn very well and this will inevitably cause the engine to run rough, produce less power or stop completely.
A tiny amount of water will generally mix in with the fuel and can reduce the power output slightly, whereas anything more will cause significant problems.
Water will get into the fuel tanks various ways, it can seep in through drains and perished seals, often this happens after the aircraft has been washed.
Condensation is also another issue. If you leave your fuel tanks with less than full fuel, condensation can often form inside the fuel tank, this will turn into water. So it is always a good idea to keep your fuel tanks full if you can.
Most importantly of all, ensure that you do a thorough pre flight inspection and always drain you fuel tanks before every flight.