Who was trying to drown Nicole and why?
My theory is that those that are trying to make "course corrections" to ensure that events happen as they should are responsible. Nicole's budding relationship with Bryce is threatening the chances that he will meet up with Keiko and get involved with her. Nicole has to be eliminated to ensure that happens.
I think that in the last episode Simon will commit suicide to ensure the second blackout (which I think is on December 12, 2016) never happens. I believe that the man who attacked Nicole is a savant who took part in Raven River, & he saw a future where, once Simon failed, it would fall to Nicole to ensure the second blackout & ultimately the end of the world happens. How Hellinger would coerce her into doing this, I don't know. Maybe he murders Bryce & convinces her Markand/or Mosaic is responsible. I also think that the man who drowns Nicole is Mason Andrews, a man mentioned on Dyson Frost's garden of forking paths. This might mean that Frost may also know of Nicole's importance, perhaps from a FlashForward where neither Demetri nor Alda killed him.
They were trying to rescue Nicole after her car veered off of the road (shown in the final episode).