As opposed to what other outcome?
Heavy, steel, iron, etc., etc. objects can reach speeds over 400 mph if they descend in sufficiently deep water. (Read about the SSNThresher.)
A massive, steel and iron (foundation or step for the anntena on the ship) assembly breaks loose from its ship and impales itelf solidly in the bottom.
Not an unlikely event--right?
400 MPH. You sure? That sounds quite a bit faster than the free fall rate through the air (terminal velocity...I think is in the neighborhood of 286 ft per second).