The gang is happily relaxing on an island when Daphne wipes out thanks to a sea serpent. They investigate the creature, called Motoshondu by the locals, and find that as usual, there is more to the situation than meets the eye.
This episode includes examples of the following tropes:
- Blind Without 'Em: Velma loses her prescription scuba goggles when a sea turtle bumps her and dives after them, asking Scooby to help her. She puts them back on to see Motoshondu right in front of her.
- Busman's Holiday: The gang ends up spending the vacation monster-hunting.
- Cassandra Truth: Velma doesn't initially believe Daphne when she claims she saw a sea serpent, claiming that statistically, it couldn't happen, and suggesting that Daphne hit her head. She changes her tune when Daphne shows the others her surfboard, which has a huge bite taken out of it.
- Chekhov M.I.A.: Crunchy, one of the locals who owns a glass-bottomed boat, goes missing midway through the episode. It turns out that he was behind the sea serpent attacks.
- Eco-Terrorist: Crunchy dressed up a minisub to look like Motoshondu and sank the tourists' boats to preserve the turtles' territory. Professor Ravenmane, the researcher from Aqualand, decries it as dangerous and irresponsible behavior that no member of the conservation community would support.
- Fine, You Can Just Wait Here Alone: Fred, Velma, and Daphne are undeterred by Motoshondu sinking their boat and put on the scuba gear to investigate further. When Shaggy and Scooby refuse, Daphne tells them they can wait up there, all alone. Shaggy refuses to fall for the paranoia, but then Motoshondu turns up.
- Friendly, Playful Dolphin: During a trip to Aqualand, Becky the orcanote gets a little too playful with Shaggy and Scooby, dragging them into the water. The oceanologist on duty brushes it off, saying she's friendly.
- Humans Are the Real Monsters: Shaggy gets spooked by a sea turtle, and Fred worries about it going back into the water while Daphne is out there. Velma tells them that the sea turtle should be more scared of humans than they should be of it and says that sometimes she believes man is the most dangerous organism.
- I Can Explain: Sharkey, one of the locals, namedrops the trope when the gang wonders what he's doing with a bunch of boat stuff, including stuff belonging to Crunchy. He tells them that it's salvage.
- Misplaced Wildlife:
- Shaggy and Scooby shock Motoshondu with a pair of electric eels. Electric eels live in South American freshwater locations, so they most likely wouldn't be in the ocean.
- Possibly also the case with the baby sea turtles, which have legs and feet like a pond turtle's rather than a marine turtle's flippers.
- Pun-Based Title: The title riffs on the tongue twister "she sells seashells down by the seashore."
- Security Cling: When the sea monster sinks their boat, the tourist family hugs each other.
- Shout-Out:
- Velma calls Shaggy and Scooby "chickens of the sea", like the tuna brand.
- Shaggy makes a quip about the sea monster being back for a scheduled feeding like Hooper with the shark.
- Surfer Dude: While surfing, Daphne is confronted by one of the locals, whose laid-back voice gets accusatory as he says she's in his spot. He's later impressed by her surfing skills.
- Tempting Fate: The father in the Cold Opening tells his wife not to worry as she tries to tell their children not to stray too far from the boat; their kids are having fun and there's nothing harmful around there. Immediately thereafter, Motoshondu shows up and sinks their boat.
- Underequipped Charge: After Motoshondu sinks their boat, the gang gets rescued by Sharkey dropping them a ladder from his helicopter. Fred tells the others that he'll keep the sea monster away while they climb up. Fred is armed with nothing but the broken flagstaff from the boat...but luckily, Motoshondu has already left.
- Yank the Dog's Chain: Shaggy is looking forward to resuming their vacation when they wrap up the mystery...only for it to turn out that their timeshare has elapsed and another family has the condo.
