Water gets into fuel through condensation in the tank, a leaking filler cap or breather, or bad fuel from the pump. Water is heavier than fuel, so it settles at the bottom of the tank where the pickup is, and the engine stalls or won't start. A clear water separating fuel filter shows water in the fuel before it gets to the engine.

When I went to start the Evinrude 150 for the first time, I found water in the fuel, so I cleared it out of the tank and fuel lines and changed the oil. In the video above, the Tohatsu 6HP on Red Dwarf stalls on the way to the mainland, and the cause turns out to be water in the fuel tank.