Obviously that isn't going to happen on water - so the impact may be harder than when landing on land.
It may be harder but the breaking back thing is a bit of a myth, I believe one guy did suffer a broken bone in the Hudson River, was able to swim ashore, and made a complete recovery, most on water since have been completely without injury.
Some seem to float a while, this one went down very quick and shows the importance of getting out damn quick