This episode marks the debut of James MacArthur as Danny "Danno" Williams, in the Pilot the role was played by Tim O'Kelly.
This show marks the debut of Richard Denning as the governor. In the pilot, it was played by Lew Ayres. During Denning's tenure in the series, he did not have a written contract. When his role was scripted he was notified to report for filming. But if he was unavailable, then McGarrett would communicate with him by telephone.
Coincidentally three of the actors who played primary characters in this early episode of Hawaii Five-O all died within a very short time span of each other: Kevin McCarthy (Victor Reese) on September 11, 2010, James MacArthur (Danny Williams) on October 28, 2010 and Patricia Smith (Joyce Weber) on January 2, 2011.
Full Fathom Five refers to a song sung by Ariel in Shakespere's play "The Tempest" Kevin McCarthy gives a rendition of it at the beginning of the episode after he dumps the body of the widow overboard. In addition to changing the gender of the subject (from a man to a woman, understandable given the context), he also makes a couple of mistakes while reciting it.
Danno finds a poison capsule in Reese's luggage. He tells McGarrett that it is "Aconite". This substance is found in Aconitum plants, also known as Monkshood or Wolfsbane. All parts of the plant are toxic; the compound aconitine is particularly deadly.