500
This is ordered by ships and boats when vessels, embarked officials, or embarked officers pass (or are passed) close aboard - 600 yards for ships, 400 yards for boats.
Such honors are exchanged between ships of the U.S. Navy, between ships of the Navy and the Coast Guard, and between U.S. and most foreign navy ships passing close aboard. "Attention" is sounded, and the hand salute is rendered by all persons in view on deck (not in ranks).
What is Passing Honors