Love your username
The fact that you are sitting "in the very back" may suggest you feel that your opinions and feelings are not as important as others. It's interesting that it is salt water in the car. Why salt instead of say, fresh? What do you think of when you think of salt or salt water?
The shark may suggest you are dealing with someone who lurks? or is untrustworthy? Or maybe even ambitious to the point of running over others who get in their way?
The reef protects you. What do you think of when you think about the reef in the dream? The reef is a barrier. Is it symbolic for the Barrier Reef? Have you been there in reality?
Anyway, you have a means of protection against forces that you feel are controlling you. It may be working to some extent because the 18-wheeler does not hit you--another symbol for something or someone you feel in danger of.