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][1] ended well [Ethiopian Airlines Flight 961 did not have the same luck.][2] You can see the outcome of [contacting asymmetrically is pretty devastating.][3]

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.][4] 


  [1]: https://en.wikipedia.org/wiki/US_Airways_Flight_1549
  [2]: https://www.youtube.com/watch?v=FCx5Uzfrb4o
  [3]: https://www.youtube.com/watch?v=WE2Yn0cipTY
  [4]: https://www.aviationsafetymagazine.com/features/the-ditching-option/