- Narrator: Cassie
Cassie and Tobias receive a message in their dreams, which appears to be coming from the ocean.
Tropes:
- Adaptational Consent: In the book, when Ax acquires the human Animorphs he says <With your permission> and touches Cassie's face without further explanation. In the comic, she has a line of dialogue where she realizes that he's asking permission to acquire her and says it's okay.
- Big Damn Heroes: A pod of humpback whales, including the one the Animorphs saved from sharks earlier in the book, arrives to ward off Visser Three before he can eat the "Andalite bandits".
- Big Eater: The seagull morphs are constantly distracted by garbage which may contain food.
- Chase Stops at Water: As humans, the Animorphs flee Controllers on the beach and end up running into the surf and morphing trout to escape. The Controllers spot their tracks and discarded outer clothing but can't tell that the kids are nearby in the water.
- Closest Thing We Got: Cassie is the "closest thing we have to an animal expert", as Jake says. She freaks out a little when he says it since at the time she's wracked with guilt and uncertainty since her insistence on trying to find the source of the message has got them to such a rough point.
- Curb-Stomp Battle: A group of whales attacking Visser Three results in a one-sided beatdown that forces him to retreat.
- Didn't Think This Through:
- Cassie morphs to squirrel because something's been getting into the barn and eating her patients and she wants to see how it gets inside. The squirrel brain overtakes her instantly, putting her in danger from not only the fox responsible, but Tobias as well.
- Marco can't swim, which is a big problem when going out to sea. When he demorphs in the water, someone else has to be present in dolphin morph to keep him at the surface, and when they're hitchhiking on a cargo ship which has a railing high above the water it's an issue - wading into the shallows like the first time isn't an option and he can't just jump over the side like the others. He has to improvise and morph part of the way while leaning on the rail, get shoved over the edge, and hope he's morphed enough not to drown when he hits.
- Ax's message was meant to reach another Andalite. An Andalite did receive it, but said Andalite is Visser Three's host, putting Ax in the Yeerk's crosshairs.
- Extinct in the Future: A potential future variation appears. Ax reveals that all species of life on Earth will be wiped out by the Yeerks, except for those required to feed their human hosts, if they succeed in taking over Earth.
- Fainting Seer: Both Cassie and Tobias receive psychic messages from a distance; when they do, they pass out simultaneously. It turns out that these messages are the thought-speak of Ax, who's trapped in the Andalites' crashed Dome Ship.
- Fantastic Flora: The trees in the Andalite dome are described as broccoli and asparagus shaped, some are blue and orange, and the water contains green crystal formations.
- Forgot I Couldn't Swim: Marco knows full well that he can't swim as a human, but he puts on a brave face and doesn't use that as an excuse not to go to sea. He learns to swim at some point between this book and Animorphs: The Escape.
- Friendly, Playful Dolphin: Morphing dolphin comes with a dolphin mentality, as is the case with all morphs, but in this case it doesn't suppress their human minds at all and they don't struggle for control. Instead the Animorphs are all inflected with joy and playfulness and swim off regarding their mission as a game. Tobias, overhead, grouses that it's like they're all six-year-olds. The dolphins become more serious when encountering sharks.
- Gaia's Vengeance: When Visser Three is chasing the heroes, Cassie calls out to the whale she'd been communicating with earlier, and two humpbacks and several sperm whales respond to fight him. Her narrations says that it seems like the sea itself is fighting back.
- Good Cannot Comprehend Evil: Cassie struggles to believe Ax when he explains that the Yeerks will destroy every part of Earth's natural ecosystem that can't be used to feed humans.
- Half the Man He Used to Be: Marco is bitten in half by a shark while in dolphin morph, forcing him to demorph in the open sea.
- Humanity Ensues: Ax morphs to human for the first time. He stumbles around because he can't figure out how to walk with two legs and no tail, plays with the human ability to make noise, and constantly turns his head because he no longer has stalk eyes to look around with.
- Lightning Bruiser:
- Visser Three’s mardrut morph is much larger than the dolphin and shark morphs the Animorphs use to travel in the ocean, and can swim almost as fast as them. While not quite as fast, it has far more stamina. He would have killed them if the whales hadn’t attacked him.
- The whales that come to the rescue of the Animorphs are even larger than the mardrut and are deceptively fast.
- Run or Die: Visser Three morphs into a mardrut, a sea creature far too large for the Animorphs to defeat with the dolphin and shark morphs they are using. All they can do is run, except the mardrut never seems to get tired while they wear themselves out after a prolonged chase.
- Screwball Squirrel: Cassie in squirrel morph feels hyperactive and very easily distracted, constantly asking "What's that??"
- Sea Monster: Visser Three morphs into a mardrut, a huge beast from an oceanic moon that propels itself through jetting out water and thrashing many tiny tails, and which can eat dolphins.
- Shapeshifter Mashup: Ax demonstrates the Frolis maneuver to acquire a human morph from the DNA of Cassie, Marco, Jake, and Rachel. The end result is an androgynous, Ambiguously Brown human boy.
- Shapeshifting Heals Wounds: Marco bitten nearly in half makes for the first time where this aspect of morphing is crucial. Previously they hadn't ever been seriously injured enough to make a note of wounds being morphed away, and they aren't certain it will happen now, or if when he returns to dolphin morph he'll be intact.
- Sixth Ranger: Ax joins the team in this book, bumping their numbers up to six, though he's not fully one of them for several more books.
- Smart Cetaceans: The dolphin morphs are noted to be intelligent, so much that Cassie wonders if morphing into a dolphin is similar to being a Yeerk in a host body. The whales they encounter are able to speak with the Animorphs while they're in dolphin form. After they save a whale from sharks, it surfaces under newly-demorphed Marco to support him.
- Starfish Language:
- Mostly thought-speech stands perfectly well for English, but Ax talking about a feature of the landscape inside of the Dome uses the word "enos ermarf", so some concepts are being used that flatly don't translate.
- The language that whales use to speak to dolphins is slow and difficult for Cassie to translate.
He spoke only in feelings, in a sort of poetry of emotion, without words. Part of it was in song. Part of it I could only sense the same way I could sense echolocation. I asked him [questions] in a language of squeaks and clicks and mind-to-mind feeling. - Threatening Shark: In dolphin morph for the first time, the Animorphs encounter sharks preying on a whale and are inspired to attack the fish. One bites Marco badly, and for the first time Shapeshifting Heals Wounds is critical.