re: Mass of the impactor
It might be very difficult to predict the effect of a large strike in the ocean. Let's hope we never get a chance to learn by observing.
Most of the ocean is about 4 km deep, but I doubt that would have much of a shock absorbing effect on the nature of the splash. Even at the minimum velocity of 11 km/sec, it would be like hitting rock. I suspect most of the kinetic energy would be converted to steam, and the tsunami would result from an explosion and water rushing in to fill the hole. Does anyone know of any theoretical studies in this area?