I'd have to side with Umbral, at least in that the effects would be a lot nastier than what we saw in the movie. My reasoning is as follows:
The Indian Ocean tsunami last year was caused by a magnitude 9.2 earthquake, the third most powerful quake on record. That wave, at its highest, was about 80 to 100 feet, and that in Aceh, less than 200 miles away from the quake's epicenter.
While we don't have exact figures for the impactor in the movie, we know that it hit somewhere off the coast of Cape Hatteras. Let's call it 500 miles from New York, for a nice round figure. The wave hitting New York was overtopping the skyscrapers, putting it at about 1000-1200 feet high at least. In other words, over ten times the height of the Indian Ocean wave at 2.5 times the distance.
Considering that the shockwaves from the Indian Ocean quake, a much smaller seismic event, were enough to cause a detectable (albeit temporary) shift in the Earth's rotation while vertically shifting the entire surface of the Earth at least 1cm, the comet fragment from Deep Impact would presumably cause similar effects on a far greater scale.
If anyone actually has the background in physics come up with some figures, though, that would definitely help; I personally don't know enough physics to do more than speculate.