...has the potential of being much deadlier than one on land, due to the possibility of tsunamis devastating populated coastal areas a great distance away from the impact site. In addition, a really large impact, even over the deeper parts of the ocean basins (the largest single environment on the planet) would not be necessarily "cushioned" by the water. The sea would essentially be a thin film of moisture covering the
impact point, like dew on a rifle target, and there would still be large amounts of pulverised rock in the atmosphere afterwards. About the only advantage to an ocean strike (this would vary enormously depending on the total kinetic energy available) would be the lack of major fires started by the sudden release of kinetic energy.

Fortunately, the earth, life, and even humanity would probably survive. Since life arose billions of years ago and prevailed in spite of innumerable strikes since, it suggests the biosphere is quite resilient. Whether our civilization would emerge intact is another matter. I don't think we can rely on technology to save us from the consequences of a major strike. I am confident that our science could help prevent one, at least in the case of periodic impactors that can can have their orbits predicted decades in advance, and which can have them modified gradually.

I think we all recognize here that developing this defense is not only within our technical capability, but that it may turn out to be one of the best possible investments we could make for our long term survival. It also has the advantage that it would provide us with the justification to develop a manned space program which would yield great benefits even if the impact strike never materializes, or at least, if it occurs in a far distant future where defense against it might be trivial.

But it is difficult, for social and economic reasons, for our global civilization to engage in long-term projects whose economic benefits are not clear, not demonstrable, and hypothetical. It simply has no short term return on investment. The resources necessary are being competed for by other projects with more obvious payoffs in the short run. And sadly, in this type of endeavor, the longer we delay getting started, the more expensive and the less effective it will be. It's the tragedy of the commons on a planetary scale.