I recorded it last night and saw maybe half of it. She can't die, can she? At first, I thought no (she IS the main character), but there's no way she suevived. Never mind cold water = hypothermia and wet chothes weighing her down - how long can a person survive underwater? She can hold her breath until she turns blue (literally, in this case), but her survival instinct will force her to inhale eventually - and once she inhales water, she's pretty much gone, isn't she? Unless, of course, McDreamy got to her in time, which I don't think he did (it took him too long to find her).
That said, on Deaparate Housewives, one of main characters killed herself in the first episode and continues to narrate from beyond the grave. Is that what's happening here?