I've read that the worst place for a Chicxulub class impact (relative to land, sea, or coast) is in shallow water, i.e. continental shelf. Coastal waters, that is.
Why? Because there's water, and there's rock without too much water on top of it.
The rock will be blasted into partial orbits, with the "burning sky" effect (everything on earth will be under an oven broiler for an hour, producing a planet-wide firestorm), dust blasted into the stratosphere (which, combined with soot and ash from the fires, will result in an "asteroid winter" as well as the sulfuric rain mentioned in another answer), and sea water quenching the glowing rock at the actual impact site will cause global cloud cover almost instantly, resulting in flooding, rain to carry the sulfuric acid, massive erosion, and general mayhem.
Deep water shelters the rock below, unless the asteroid involved is much larger than a few kilometers, and land lacks the prompt cloud effects and planetary rains.
From comments, there's also this paper, which suggests that the angle of impact also contributed -- by way, once again, of maximizing the mass of material thrown into and above the atmosphere, as well as its dispersion.