[wolfcreek]

If the one that hit the Yucatan about the time of the demise of the dinos, created a big enough and deep enough crater, I would assume magma and gases would have been released.

[NormsRevenge]

I don’t know in that instance how much of a penetration into the crust much less the outer or inner mantle occurred,, but it would sure seem like stuff would come popping up and out of that general vicinity.

[null and void]

Worse possible scenario. Shallow water impact. Not only all the crustal damage of a dry land strike, but the instant release of massive amounts of carbon dioxide from a mile thick layer of near shore carbonates laid down as the shells of sea life. Not only the crustal damage, and CO₂ release, but a huge tsunami wiping out shoreline life for thousands of miles. Not just the crustal damage, CO₂ release, and tsunamis, but a global rain of red hot ejecta.

Then there's the really bad part.

The ocean rushes in and tries to fill a 100 km wide white hot crater.

Now instead of passively radiating the excess heat out into space like a dry land strike would, the crater flashes untold cubic kilometers of salt water into steam. Not only does this pretty much steam clean the entire earth's surface, it also rains salt out onto everything, so most plants can't grow even if their seeds managed to survive all the above excitement.