It's very important for ship captains not to hit anything. Shell plating is surprisingly fragile. You mentioned a speed of 70 mph in the context of an object striking the surface of the water. You're not wrong about what happens in that case, but it's not very relevant to understanding the kinds of collision we're dealing with here.
SS Park Victory struck the rocks near Finland at an unknown speed. However, the reports indicate she was dragging her anchor in a strong gale at the time. This will naturally limit her speed. Further, the more extensive reports indicate she was battered repeatedly against the rocks by wind and waves. That's not quite the same scenario as a single strike.
Similarly,
MS Costa Concordia struck the rocks near Gigli, Italy at 16 knots. That was enough to puncture her hull. And of course she was moving through the water at the time, dealing with water resistance at least as it affected the underwater portion of the hull.
16 kts is on the order of 8 m/s. The reference speed in our car example comes out to about 20 kts. Intuitively we can quite easily say that a ship that hits a rock while going 20 kts won't suprise anyone by suffering a hull breach.
@Andy_Ross has posted a number of photographs of ships that suffered fractures in hull plating under normal operating speeds and water conditions.
Now that we have shown that vertical and horizontal collisions result in the same kinetic energy (although arrived at by different means) we can equate horizontal collisions between ships and rocks with vertical collisions between ships and rocks. All we need to know is the kinetic energy at the point of collision in order to continue our reasoning. The next exciting chapter will be whether a vertical velocity of 8 m/s makes sense for a sinking ship.
