Look at the street view, the water is very close to the road. They would not need to have been going that fast. Just one facebook message at 70 kph and they would have been in the drink.
It is possible that they went through the light, drove over a bit of grass, crashed through a fence, over more grass, then splash.
Unless this was 2am I'm not sure how they could do that without anyone seeing! Wouldn't someone notice a car doing all of the above?
Even if it happened without someone seeing, wouldn't someone call the police later about a giant flattened piece of fence with tire tracks going to it?
Seems too suspicious to be an accident. This sounds like it was on purpose to me.
The tire tracks into the lake will give the answer. Sounds like they met someone nearby then they were pushed into the lake or something.