For impacts on the scale of Tunguska or Meteor Crater -- tens of megatons -- the destructive effects are only local. Even more true if a high altitude airburst. Hiroshama-scale impacts are certainly not going to have any effects at all at a distance.

We've conducted a number of oceanic surface bursts in the 10-20MT range, and outside of the local area (i.e., within 100km or so) the only real threat is radioactive fallout, which won't be an issue with an impact. Maybe flash blindness for a night explosion and dark-adapted individuals. I don't even think you get an EMP effect since that has to do with x-ray induced ionization over a very wide range at very high altitudes. I think -- welcome corrections -- the plasma tube an impactor creates would have far smaller ionospheric effects than a high-alt nuclear blast of the same total energy release.

For impactors of km scale, you're right. But for the sorts of impacts we expect annually or even once in 100,000 years, ocean hits are much better!