Question about Charlie st. cloud movie?

So i saw charlie st. cloud yesterday for the first time and it was
beautiful. i couldn't stop crying. if you haven't seen it i totally
recommend that you do. but for those who have seen it. i have a few
questions about the movie. there were so parts that confused me.

1. during the car accident when charlie and sam were holding on to
each other, were they dead? was sam already dead? was it a dream or
was it actually happening?

2. and in the ending when charlie visits sam for the last time what
promise do they make?
and why does sam say "come rain or shine. hell or high water"?

thanks