[blam]

"As bad as the death and destruction from coastal inundation would be, it's slowly dawning upon me that it would just be the beginning."

Yup. Have you considered the 'cosmic winter' that is sure to follow from all the sulfuric acid droplets blocking the sun for years.

[null and void]

It's worse than that. This was a shallow water impact, shallow enough so that the earth's crust was cracked down to the mantle.

Now, instead of the white hot dry land crater radiating most of its' heat out into space over a period of days or weeks, and unlike a deep water impact where the depth of the water absorbs and spreads the blow, and the steam generation is limited to the kinetic energy of the bolide, we have a wall of sea water attempting to flood into a crater that has not just the energy load from the impact, but all the heat of a 100 km wide exposure of magma.

It doesn't take much imagination to envision a scores of fathoms high, hundred mile wide, circular wall of water attempting to fill a white hot crater and being flashed into steam, be blown back, only to surge again in an lethal, eerie inverted waterfall as the leading edge is blown into the stratosphere by a blast furnace of live steam.

As cubic mile after cubic mile after cubic mile of ocean boils, the resulting global hurricane rapidly dumps all that heat over nearly the entire planet. Anything within half a planet from the crater would be steam cleaned. The rest would merely be parboiled.

It's the recipe for pasteurized planet...