On What True Story Is "Open Water 2: Adrift" Based?

The original open Water was based on the true story of Tom and Eileen Lonergan were left behind by their diving party after diving off the Great Barrier Reef in 1998.
The sequel is, as mentioned previously, actually based on the short story "Adrift" by Koji Suzuki.
I only know about the first one as I'm Australian and it made headlines everywhere here.
Hope this helps.

OPEN WATER 2 isn't based on a true story. It's based on the short story ADRIFT which is from a collection of short stories called DARK WATER (which is also where the movie DARK WATER starring Jennifer Connelly comes from)

Well I just saw this movie last night and it indicates in the beginning that "This story is based on true events"!

It is not really based on a true story.
It is actually based on the short story "Adrift" by Koji Suzuki, from the novel Dark Water

I have not yet seen the movie you speak of, however, I have seen the original movie entitled, Open Water, which was based on a true story of a couple who vacationed in the Bahamas and met their demise after being erroneously left at sea on a dive trip.  I hope this helps you sweetie...nassy