If you're still inside the plane when it crashes into water you can usually survive because the plane will protect you from the direct hit into the water.
However if you do not get out of the plane and water in time then you could die from the plane crash into the water due to hypothermia.
The water in oceans and the sea is usually much too cold for the human body to withstand for too long without getting hypothermia.
Most people who die as a result of a plane crash into the water dies of hypothermia because the water is too cold for the persons body.
But if you're rescued from the water in time then you will likely survive when crashing into the water in a plane.
There have been some plane crashes into the water where everyone on board the plane has survived so yes it's possible to survive a plane crash into the water.
Crashing the plane into the water is much safer than crashing the plane directly into the ground because a plane will likely catch on fire or you'll die because of the force of the impact of the plane onto the ground.
So crashing the plane into the water is usually the much better option if you have no other choice but to crash the plane.