Although, he's probably been giving half-truths this whole season, I'm inclined to believe he is telling the truth about NOT being able to become smoke and travel across the water. In fact, that reveal came the same episode when Zoe asked Jin to show them the pockets of EM energy around the island. I think that was purposeful, as if the source of the smoke monster's form has something to do with him being "tied" to the physical land of the island, and thereby the energy pockets. Hence Widmore needing Desmond for some reason. I could be wrong. But there aren't any pockets - presumably - in the water. And besides, if he could turn to smoke over water, why bother to go to Hydra island for a plane, he could just fly away to the mainland or wherever "home" is.

Also, when "Christian" appeared to Michael, the whispers were heard. And in spite of what dead-Michael told Hurley last week, the whispers up until that point almost always accompanied the appearance of the Others (i.e., Harper appearing to Juliet, the Others taking Cindy the stewardess, Ben telling a young Rousseau that if she ever hears whispers to run the other way). Methinks that the "Christian" that appeared to Michael was something else.

My theory is that the Man in Black, Jacob (?), and something else (?) have all been using the "Christian" identity at various points. We still don't know who else Hurley saw (the eye peeking through the window) in Jacob's cabin besides Christian.

We'll see.