It's just error of judgment - caused by alcohol. They've had a night out, decided to jump into the sea for a laugh not realising how strong the tides are or how dangerous it is. It can be very deceptive.
It's on the Argus site now (they've finally cottoned on to the fact there is a big local story despite it being on the national news hours ago)
http://www.theargus.co.uk/news/10893628.BREAKING__Woman_believed_to_have_been_swept_out_to_sea_in_Brighton/?ref=var_0