I have seen a map that shows where the car started from and where it ended up. To get to the ferry taking them to Edgartown (where they were going) they would have to have turned left at an intersection. It was late and very dark and deserted. For some reason the car turned right and headed towards the embankment where it went into the water. The bridge there only takes you to a thin strip of beach (where you need to deflate your tires in order to drive on it). It makes sense to me that the turn left was either his mistake because he was too drunk to know better or that he wasn't in the car at the time. There was no reason for Kennedy and Kopechne to go to the barrier strip.
Chappaquiddick is not at all well lit. It is and has been very rustic and secluded. Only 30% of Martha's Vineyard is paved to this day! That's how they want it there...here is a map so you can envision it better
http://maps.google.com/maps?hl=en&tab=wl. You have to enter "Chappaquiddick" in the Search box, tho...sorry.