At the AGU 2010 Fall meeting, the Science & Entertainment Exchange hosted a panel on Hollywood science. On it, screenwriter for Deep Impact, Bruce Joel Rubin, revealed that he got a bit carried away doing science research while writing. For anyone who missed the movie when it came out, a major impact hits the Earth, sending out waves of devastation including a tsunami.
Tsunami are formed by any major displacement of water. Meteorite impacts into oceans displace huge quantities of water, sending out a tsunami. Historically, this may have happened with the Cretaceous-Tertiary extinction event.
In open ocean, where water is a few kilometers deep, tsunami are visually undetectable, with a small wave height (10 cm to 1 m) spread over an enormous wavelength (200 to 400 km). Instead, they are detected via pressure changes on the ocean floor. As a tsunami enters shallower water, the wave shoals, reducing in wavelength and increasing in height.
Shallower water begins at the continental shelf, which may be hundreds of kilometers off shore, so tsunami may be visible (and cause damage) offshore. Shane Wanless points out this image is reinforced in the movie by a scene of an offshore oil platform destroyed by the incoming tsunami. Although floating oil platforms can be in incredibly deep water, fixed platforms are in relatively shallow water depths of up to a few hundred meters. For a bit of bonus good science, Deep Impact successfully depicted a tsunami near shore as a wall of water (akin to an incredibly rapidly rising tide) instead of the more typical Hollywood image of a plunging breaker perfect for surfing.