Why isn't landing in the water a good option in an emergency?
For one you can not "land on water" you can ditch an aircraft in a body of water but you would be hard pressed to call it a landing. You can however, "land in a grass field" or "land on a runway nearby" or "land on a highway" all of which are substantially more controllable then trying to stall a few feet above the ocean.

The result of catching an edge which can dig into the water is the same or worse than belly landing on grass. As others have mentioned at those speeds water is not so forgiving. but historically it tends to end poorly. While the miracle on the Hudson ended well Ethiopian Airlines Flight 961 did not have the same luck. You can see the outcome of contacting asymmetrically is pretty devastating.

The secondary concern is that even in a successful ditching still puts you in a sinking airplane with potentially injured people making rescue all that much harder.  

Although Aviation Saftey Magazine paints an interesting counter point.