The chances are an asteroid hitting the earth would splash into an ocean or a virtually uninhabited area like the Sahara or Siberia. We should wait until the time and place of impact can be calculated more precisely. If it's going to hit someplace valuable, try to deflect it. If it's going to hit a wasteland or Iran, leave it alone.
If a sizable asteroid splashed into the ocean, the resulting tsunamis would likely kill millions, among many horrific side-effects.