Actually, apparently some overzealous passenger did begin to open a tail exit slightly before being stopped by a cabin crew member and re-closed. I believe that water did enter the cabin from that door while it was open. Well, considering that everyone survived, that wasn't a catastrophic mistake.

The passenger obviously made a huge mistake because the first thing to do when reaching an unopened exit is to look though the window for any hazards, like fire or being underwater.

This is purely my own thinking, but it doesn't seem to make sense for the cabin crew to advise everyone to head to the tail unless the plane was noticeably tilted nose down and was underwater. In this case, since the tail section was submerged and not the nose section, reports that the crew advised passengers to the tail doesn't seem logical to me, unless it was an early command for passengers in the tail section to head for the tail exits since those are the closest exit for that particular section.