Just watched this the other night, I never saw open water but they say the premise is the same; cap-sized boat, 5 people in the ocean, getting hunted down by a great white.
I will say it hit on one of my greatest fears (being alone in the ocean and getting attacked by sharks) but I really enjoyed the way this movie was shot and it had some of really believable shark attack shots.
All in all I would recommend this if your interested in shark attack movies, it was very well paced and was a pretty good thriller.