What happens if a giant asteroid strikes the ocean?

From Gizmodo, an article by Maddie Stone (with the usual unbloggable video) about asteroid disaster:

Seventy percent of the Earth’s surface is covered by water, meaning that, if we were unfortunate enough to be struck by an enormous asteroid, it’d probably make a big splash. A team of data scientists at Los Alamos National Laboratory (LANL) recently decided to model what would happen if an asteroid struck the sea. Despite the apocalyptic subject matter, the results are quite beautiful.
Galen Gisler and his colleagues at LANL are using supercomputers to visualize how the kinetic energy of a fast-moving space rock would be transferred to the ocean on impact. The results, which Gisler presented at a recent American Geophysical Union meeting, may come as a surprise to those who grew up on disaster movies like Deep Impact. Asteroids are point sources, and it turns out waves generated by point sources diminish rapidly, rather than growing more ferocious as they cover hundreds of miles to swallow New York City.
The bigger concern, in most asteroid-on-ocean situations, is water vapor. “The most significant effect of an impact into the ocean is the injection of water vapor into the stratosphere, with possible climate effects” Gisler said. Indeed, Gisler’s simulations show that a large (250 meter) rock coming in very hot could vaporize up to 250 metric megatons of water. Lofted into the troposphere, that water vapor would rain out fairly quickly. But water vapor that makes it all the way up to the stratosphere can stay there for a while and, because it’s a potent greenhouse gas, this could have a major effect on our climate.
Of course, not all asteroids make it to the surface at all. Smaller sized ones, which are much more common in our solar neighborhood, tend to explode while they’re still in the sky, creating a pressure wave that propagates outwards in all directions. Gisler’s models show that when these “airburst” asteroids strike over the ocean, they produce less stratospheric water vapor, and smaller waves. “The airburst considerably mitigates the effect on the water,” he said.
Overall, Gisler says, asteroids over the ocean pose less of a danger to humans than asteroids over the land. There’s one big exception, however, and that’s asteroids that strike near a coastline.
“An impact or an airburst near a populated shore will be very dangerous,” Gisler said. In that case, the gigantic, city-devouring tsunami every B-list disaster movie has primed you for might actually arrive.

Rico says he forwarded the article to his father, a world-famous oceanographer at Scripps Institution for Oceanography, for comment...