Hallo Marina, 'crashed' means to collide violently with an obstacle - in this case the water, which, from the height and at the speed with which an aeroplane comes down, is usually like hitting a piece of concrete and results in the aircraft breaking up.
'plunge' into the water, would not be the same, because it does not indicate the breaking up of the object, but rather a smoother entry. I can't think of another word that has the same all encompassing meaning as 'crash'.

:-)) Thanks. In fact I don't like 'crashed' either, when it is into the water. But it seems to be commonly used.

Marina, may I suggest that 'crashed into the sea off the coast of Vera Cruz.' would be more correct. 'at sea' indicates rather that the plane was travelling in the water.