On the island of Kavekana, Kai builds gods to order, then hands them to others to maintain. Her creations aren't conscious and lack their own wills and voices, but they accept sacrifices, and protect their worshippers from other gods—perfect vehicles for Craftsmen and Craftswomen operating in the divinely controlled Old World. When Kai sees one of her creations dying and tries to save her, she's grievously injured—then sidelined from the business entirely, her near-suicidal rescue attempt offered up as proof of her instability. But when Kai gets tired of hearing her boss, her coworkers, and her ex-boyfriend call her crazy, and starts digging into the reasons her creations die, she uncovers a conspiracy of silence and fear—which will crush her, if Kai can't stop it first.
Full Fathom Five provides examples of the following tropes:
- Chekhov's Gun: At one point, Kai offhandedly mentions that she's trans. This becomes important later when she is trapped inside a Penitent. Because she's already used to the whole world telling her she's something she's not, she can resist it better than most.
- The scar that Teo has from nearly being sacrificed by Temoc in Two Serpents Rise - it's what helps her escape from her Penitent.
- Deity of Human Origin: The priests of Kavekana make their profession out of building idols, not-quite-sentient mini-gods that fulfill the same purpose as offshore bank accounts for wealthy clients. The highest tiers of the priesthood have a vested interest in covering up the fact that putting all those mini-gods together ended up accidentally bootstrapping a real god into existence.
- Disabled Deity: Mako. He was badly wounded in the God Wars and is lucky to have survived in the first place, so he hasn't got the power to fix himself.
- Establishing Character Moment: In the very first chapter, Kai dives into the pool to save an idol who's scheduled to die because she thinks she can save it (and nearly manages to do so), showing her to be brave, skilled, stubborn, and slightly reckless.
- Gods Need Prayer Badly: The more Izza tells stories about the Lady and gets the other children to leave offerings for her, the stronger she gets.
- God in Human Form: Mako, aka Makawe, was trapped in a human body after nearly dying in the Wars.
- God Was My Copilot: Mako is Makawe, former chief god of Kavekana.
- Illegal Religion: For a completely different reason than the previous book. Other gods are banned from Kavekana because it's bad for business.
- Loophole Abuse: Jace bars Kai from the mountain and the idol pool...in her capacity as a priest. However, in her new position as a facilitator between pilgrims and idols, it's perfectly acceptable for her to show the pool to clients.
- Mind Rape: Criminals are locked inside the Penitents, massive stone automatons that force the people inside to think like law-abiding citizens. After a couple years of being painfully trapped inside the Penitents, either fighting crime or just staring at the sea waiting for the gods to come back, people come out very, very quiet and very, very unlikely to break a rule ever again. It happens to Kai, as well as Mara.
- Prophecy Twist: Before the beginning of the God Wars, there was a prophecy that the gods would return bearing the worlds treasures and the islands would be crowned with light. People took this to mean the gods were going to win—not so much. However, the prophecy still comes true. The only god to return was Mako, aka Makawe, and the islands are "crowned with light" when Cat triggers the flashbang spells she drew all over. The treasure is all the money Kavekana holds as an international tax haven for the rich and powerful.
- Street Urchin: Izza and her friends.
- Well-Intentioned Extremist: Jace truly believes that killing the idols and forcing people into Penitents helps Kavekana.