What if a comet hit the Earth?

COMET HITS EARTH AT DEEP IMPACT! <------- Press to watch

This is one of my favorite astronomical YouTube videos because it was made to resemble a movie trailer. This video is very interesting because it shows what can actually happen to the earth if it was hit by a comet at deep impact. I would not want to be on Earth if a comet was about to hit. I am not  sure if a comet moving at that speed and size would cause that much impact, this video might be an over dramatization. Could a comet do that much damage to Earth? Can the water envelop all of a huge city like New York and ruin everything? Is it possible in the future that a comet like this can hit Earth and destroy all of Earth? What would be the precautions? -- it is most definitely a possibility.
The water rising and destroying everything including all of New York (HUGE BUILDINGS) was fascinating and scary at the same time! Hopefully no comets are planning to hit Earth anytime soon!

What is a Comet: Read more, got information from here
Summary: Comets are often referred to as "dirty snowballs." They are left over from the formation of stars and planets billions of years ago. Before zipping around the Sun with their characteristic big tails, comets that we see in our solar system start out as big chunks of rock and ice just floating around in something called the Oort Cloud. When the gravity from a large passing body, like a star, becomes strong enough, some large chunks of ice get pulled away from the cloud and head toward the Sun. As that ball of ice gets close enough to the Sun, its heat begins to melt some of the ice that makes up the comet. The melted ice becomes a gaseous tail that extends away from the source of the heat (in this case, the Sun). The tail is pushed out by the Sun's solar wind.