Roots: The Perfect Storm. Mermaid folklore and Greek myth: Nereus is father of the sea-nymphs, Amphitrite was the wife of Poseidon; Hesiod's Theogeny includes a nereid called Galateia; The Doctor mentions Hans Christian Andersen's The Little Mermaid, Star Trek ("don't call me Bones..."), Todd Browning's Freaks.
Dialogue Triumphs: "You know something, don't you?" "I know lots of things. I'm an almanac. With wit"
Double Entendres: "When all this is over we can go back there and you can take as long as you like staring at the baobabs"
"He's probably shivering his timber right now, or whatever it is you sailors do."
Continuity: The Merpeople species is indigenous to Earth and endangered (the Doctor sees hundreds below water). Merpeople have outer and inner eyelids and scaled tails, and can breathe air for a time when mature. Some have the ability to walk on land, but this requires a great deal of strength - Nerus demonstrates an ability to transform his own body, changing his tail into human-like legs. Amy is human in appearance above the waist. Their 'home is the ocean floor, a colourful desert of brimstone that they float above while bathing in its warmth. Their homes are made of coral with roofs of mussel shell that open and close with the flow of the water. Their weapons are also of coral, but specially strengthened and weighted by means unknown to the Doctor.
It may be inferred via De Requin's code-word "For King and Country", that the collection of a living Merperson has been aided in some way by The Forge.
While the Ship is on deck salt water begins to eat away at the outer shell of the TARDIS.
Originally intending to partake in a sea cruise, the Doctor and Peri have been onboard the Lankester for at least six days (the duration of the storm currently besieging the vessel is currently four, while the storm hit two days out from the Cape of Good Hope, although conditions were calm when they boarded), coming aboard while it was moored in Madagascar, in a port south of Nosy Barren. The Doctor admits that his marine biology is a bit rusty, and thought Merpeople simply a myth. He claims he only has a swimming certificate "for doggy paddle."
Untelevised Adventure: The Doctor tells an anecdote that suggests he met Livingstone.
Links: Storm Warning (the Doctor says the smelling salts "would wake a sleeping vortisaur")
Q.v. 'For King and Country: The Forge'
Location: the cargo ship Lankester the Atlantic Ocean midway between Madagascar and New Orleans, 20th of October, 1901
The Bottom Line: "Back you go... make a splash!"
A harmless first effort and something of a bar-setter for future subscriber-bonus stories. Starting the story of in media res means the villain of the piece is quite obvious without the Doctor and Peri flagging him as well (plus he has the most bonkers laugh.)