The gang are hanging out on Cher's yacht for Scooby's birthday when their mystery-related luck strikes again. Two shark-men find their way on board and steal the ship.
This episode includes examples of the following tropes:
- A Birthday, Not a Break: Scooby's birthday celebration gets interrupted by another monster mystery.
- Chekhov's Gun:
- The gang finds a piece of deep-water seaweed on the cabin floor. It turns out that the captain and first mate have been diving to plunder a famous sunken treasure ship.
- Further investigation turns up a wet flipper-print in the cargo hold. The shark-men were stowing the gold from the Dolly in the cargo hold after bringing it up from the wreckage.
- Chekhov M.I.A.: The captain and first mate disappear after the shark man comes on board. It turns out that the two of them were posing as shark men to scare everyone off and loot a shipwreck.
- Chekhov's Skill: Cher shows her fondness for beading early in the episode. She and the gang end up using the beads as part of the trap.
- Disguised in Drag: Shaggy and Scooby serve as bait (as usual) while dressed as mermaids.
- Foreshadowing: The shark men have no trouble sailing the yacht after stealing it when the gang and Cher go to investigate the wreck of the Dolly. This makes sense given that they're the crew.
- Green Aesop: Downplayed; Cher mentions early in the episode that her yacht is equipped to gather plastic from the ocean and turn it into beads. However, it doesn't come up afterwards.
- I Gave My Word: Cher promised Scooby a birthday cake, and nothing, from Shaggy and Scooby eating most of the batter to a pair of shark-men running around, is going to make her break that promise.
- Immediate Self-Contradiction: When Fred, Velma, and Daphne come to see the bridge, Daphne mentions that Fred's been waiting all day for this. Fred claims that she's exaggerating, only to undercut himself immediately by geeking out over everything in the room, including a fairly ordinary phone.
- It's the Best Whatever, Ever!: Fred, who's been eager to see the bridge all day, gets the chance to sail the yacht himself for a while. He enthuses that this is the best day ever.
- Man-Eating Plant: Velma mentions the legend that the seaweed from the titular sea is carnivorous and eats the sailors whose ships get stuck in it. Cher isn't happy with her bringing it up. When the captain turns up missing with a piece of seaweed in the cabin, Shaggy freaks, thinking that the seaweed ate him.
- Mythology Gag: Cher mentions encountering another shark monster the first time she met the Scooby gang, a reference to the "Mystery on Shark Island" episode of The New Scooby-Doo Movies.
- Never My Fault: Cher calls out the culprits after they pull the standard You Meddling Kids speech, asking how it's supposed to be her fault that they got caught for crimes they committed.
- Nightmare Fetishist: Velma considered the incident at Shark Island where they first met Cher "fun". Cher dryly comments that they have different definitions of the word.
- Self-Deprecation: Played With; after the gang does their Once an Episode chorus of her name and accomplishments, Cher tells them it's unnecessary. However, she admits she likes it.
- Shark Fin of Doom: Cher first sights the shark man when she spies a fin slicing through the water behind Shaggy and Scooby as they're in the pool. Before long, the rest of the shark man follows.
- Shark Man: The villains are sharks with animal-like legs and arms and they're called shark-men.
- Wealthy Yacht Owner: Superstar Cher has her own yacht, where the majority of the plot takes place. Upon being trapped, the culprits say that they wanted their own yachts to use during a wealthy retirement.
- What Happened to the Mouse?: The chef who is supposed to make Scooby's cake disappears after being attacked by the shark man during the opening. No one ever mentions what happened to him.
