A semi-analytical model of impact heating and shock isobar geometry based on computer code calculations has been developed and used to estimate vaporization of water in oceanic impacts. The mass of water vaporized in ocean impacts by projectiles of asteroidal (iron, rock) and cometary (ice) compositions is found to: 1) increase as roughly the square of the impact velocity above a threshold velocity, and 2) to be about an order of magnitude larger than the amount of rock vaporized in similar continental impacts. The mass of water vaporized in an infinitely deep ocean by the impact of a 10 kilometer diameter asteroid at 25 km/sec—the nominal diameter and impact velocity of the proposed Cretaceous-Tertiary extinction bolide—is roughly equal to the total mass of water vapor typically present in the earth’s atmosphere, and three to four orders of magnitude larger than the mass of water vapor in the stratosphere. The global climatological effects that may be attributed to such a massive and geologically instantaneous injection of water vapor into the atmosphere may be significantly reduced by two factors: first, the point-source nature of an impact implies local water vapor super-saturation of the atmosphere, causing much, if not most, of the vapor to condense and rain out before it can be transported throughout the entire atmosphere. Second, for projectiles of multi-kilometer dimensions, the finite depth of the ocean becomes important and may significantly reduce the amount of water vapor initially produced in the impact. The strong dependence of water vapor production in oceanic impacts on projectile size, composition and velocity, and on the ratio of the projectile diameter to the depth of the ocean at the point of impact implies that climatological models and extinction scenarios invoking the effects of impact-generated water vapor may depend critically upon the a priori ill-defined details of the hypothesized impact.