Sometimes you can survive jumping from a plane into the water and sometimes you could die.
It depends on how high the plane was before you jumped from the plane into the water.
If the plane is 250 feet or more in the air and you jump then you'll likely not survive the fall into the water since falling into the water at that height can cause you to rupture vital organs and break bones upon impact with the water.
If the plane is below 250 feet then you could still die from jumping into the water from the plane but your chance of survival is much better.
Either way if a plane is crashing you might die either way and I'd rather die jumping out of a plane instead of dying in the plane.
At least I'd be having fun going out. :)