[...]visibility was clearly quite bad at the time of the accident so it's hard to fault the driver.
It is the responsibility of the driver to adjust his driving to the environment. If he cannot stop in time to avoid a stationary object, then he was clearly at fault - whether he was driving too fast or was somehow distracted.
Sure, he may have had no reason to assume there was a lake in front of him. Every time I drive, I keep in mind that there may be a vehicle, person, animal, road damage, a snow drift, fallen trees/rocks or some other object in my lane. When it rains or snows, I drive as if the road is slippery. When it's dark, foggy or snowing/raining hard enough to effect visibility, I drive only as fast as I can see.
Regardless of whether the GPS is found to be liable, above all else, it is the responsibility of the driver to drive safely.
@Jane: I tend to like your articles, but I pray that you don't have a driver's license.