Most modern extension cord socket ends are deep enough and narrow enough that the surface tension of water won't break past the opening or aperture. Contaminants in the water can mitigate the surface tension a bit (surfactant runoff from stuff like anitfreeze, pesticides and herbicides etc). The chance that dirty water can get to the actual electrical contact is minimal, but it is possible. If the cord is plugged into a gfdc, then it will likely trip out and open the circuit before anyone gets hurt. If no gfdc, then the contacts could short out through the water and trip the breaker. There's also a very remote chance that the water is not conductive enough for a true short, for whatever reason, and the current stays live. Usually, though, once the current has found a flow path in the liquid, it will usually compel a circuit flow to a true short or open the circuit again.

It takes a serious case of dumb butt to leave a cord thus.