i bet the reason that juliette is there on the beach is to steal sun since she's pregnant and they want to do tests on her, 'cause all othe other pregnant women (other than claire) died... and why did julliette agree to do this? because ben told her that the sub wasn't the real way to get off the island, and that transport to and from the island was actually via another way (maybe the "magic box" from 2 episodes ago?). in the flashback, it was just too convenient that she woke up right when they arrived at the island. the sub was just a way to fool her, and keep other people there, thinking they could leave anytime on it. i bet ben said "steal sun, check out what's up with her preganancy and i'll really let you leave"... anyway saide will eventually have his way with her.

edit: and i bet jack knows that juliette is up to no good, and is telling everyone he trusts her so she lets her guard down. jack isn't that dumb.