Here's my theory, for what it's worth (great part by the way!)
Lois told Lex that she wanted a divorce. He said she'd have to be dead first before he'd let her divorce him, so she decided to fake her death.
Except that it got out of hand, Lois didn't realize how cold the water was, etc etc etc, and she got in over her head (pardon the pun).
Please - don't let Clark be kidding when he says he flew. It's about time he shared his secret with the intrepid Lois Lane! After all...she has a secret, too.
I think a nice fire inside a certain Smallville farmhouse is just the place to ensure Lois doesn't get pneumonia!
