Almost three quarters of the planet is ocean, so for every object that strikes land, about three should hit the sea. Marine geologists are now reevaluating evidence of tsunamis that may be too big to come from undersea volcanic eruptions. Some could be the result of a kilometer-wide object plunging into the ocean at 20 miles per second.
Randall's larger point is about events like Tunguska, which leave no lasting crater. Those events are far more abundant than the impacts that dig holes in the ground. Count only craters and scars, and you will undercount how often Earth is hit.