[evgen]

3.4 million kg, traveling at 600 m/sec, and you think you would end up with a big ball of floating nickle? Okay....

Not a physicist, but my thinking would lead me to say that when this ball hits the water (after having already been heated a bit by ablation on the way down) it is going to compress into a pancake. Assuming the energy released at impact does not break the ball into lots of smaller pieces, the energy of the impact is going to vaporize a lot of water and melt a lot of nickle. I think what you are going to end up with after the large impact event is a compressed blob of nickle that is now rapidly sinking to the floor of the ocean.

We already know what happens to big chunks of metal from outer space that impact the surface of the earth, it tends not to be pretty. Oh yeah, and try to convince any country on the planet to let you drop your giant ball of awe-inspiring kinetic energy on a path that happens to cross over them. Not. Going. To. Happen.