How many ditchings under similar circumstances - engine failure at a high enough altitude and with enough airspeed so that the crew had time to analyze and decide where to go, followed by a smooth water surface to land on, and (judging from the water surface as shown on the news last night) not much wind - have there been?
From what I can find there have only been 3 or 4 other incidents where a large jet airliner has intentionally ditched (depends on whether you count the Garuda plane).
The first one was a Overseas National Airways DC-9 that ran out of fuel after making multiple approaches to St Maarten. There were multiple failures on the crew's part and many passengers did not survive, but most did. The plane sank relatively quickly, reason unknown.
ONA/ALM Ditching
The second was the infamous Ethiopian Airlines 767 that ditched off the coast of the Comoros. The pilots were fighting with the hijackers for control of the plane (as well as being smacked with a hammer among other things) which is part of the reason why it flipped when they tried to ditch it. Apparently many people inflated their life vests inside the plane and were trapped as a result.
Ethiopian hijacking and ditching
Two others that I can find are a Garuda Airlines 737 that ditched in a river 1 meter deep in 2002 and a Russian Tu-124 that ditched in a river in 1963.
Garuda ditching
Aeroflot ditching with picture