Remember that the tail of the plane landed in the water...so Ana Lucia didn't necessarily fall from the plane into the water, but did have to swim up from the tail to the beach...
It's only the 1st episode...they usually only set things up, not necessarily explain.
Go to
4 8 15 16 23 42 - LOST Numbers TV Episodes Discussion Forum and read the Seventh Seal Theory. This guy have written a few other elaborate theories, but it's cool to see someone's imagination at work (Handle is Andrew Smith).