The results indicate that asteroids with a diameter greater than 200 metres (the length of two football pitches) will hit the surface approximately once every 160,000 years – way down on previous estimates of impacts every 2,500 years.

The findings also predict that many more asteroids blow up in the atmosphere than previous estimates, which means the hazard posed by impact-generated tidal waves or tsunamis is lower than previous predictions. The researchers suggest that proposals to extend monitoring of Near Earth Objects (NEO) to include much smaller objects should be reviewed.

This may explain the dearth of substantive evidence around the vast ocean basins that tsunamis have washed away coastline areas.
If on the couple thousand year timescale a big rock hit the ocean, the tsunami effects would be widespread and should have left more records somewhere.

-p (want's EPG's (sic) asteroid money for Marks Mars trip)