But in this case with slow speed, it's the massive (literally) amount of mass of the cargo ship that gives it an un-intuitively large amount of energy.

8kn isn't super slow

It's 10 mph which is pretty slow as speeds go.

Slow-but-irresistible force meets movable object.

All objects are surprisingly movable when they encounter enough mass.

To your point, it is mass. Most people don't understand mass on water. At 1 knot the ship would do the exact same damage, topple the support and drop the bridge.

Motor control and rudder control in a momentumous frame of reference.

sqrt of 100K times weight of a car is 300 cars worth.

Nitpick: squaring makes numbers greater than 1 bigger, and numbers smaller than 1 smaller. In this case we're squaring something with units, and we can't say the input is greater or smaller than the output, because they have different units. What we can say, is that v^2 curves upward: it grows faster with increasing v, and slower with decreasing v.