The Horn of Time the Hunter, by Poul Anderson

A story of spacefarers, removed from planetary time by interstellar travel, encountering a civilization on a watery planet. This story comes from Poul Anderson's short story collection, The Horn of Time , first printing by Signet in January 1968, copyright Poul Anderson. The story was originally published in Amazing Stories (Ziff-Davis Publishing, September 1963) under the title, "Homo Aquaticus."