The movie has received mostly received mixed-to-negative reviews from moviegoers and critics alike and has even gone down as one of Dreamworks' weakest films.
In Reef City, an underachieving bluestreak cleaner wrasse named Oscar fantasizes about being rich and famous while making his way to work as a tongue scrubber at the local Whale Wash, a job in which he is following in his father's footsteps. Soon after arriving he is called to the office of his boss, a pufferfish named Sykes, to discuss the fact that he owes "five thousand clams" and has to pay it back by the next day. After explaining this to his angelfish best friend (although way omre of his love interest than sister figure) Angie, she offers him a chance to pay back the money by pawning a pink pearl that was a gift from her grandmother. Oscar brings the money to the race track to meet Sykes, but becomes distracted by his dreams of grandeur. Upon hearing that the race is rigged, he places it all on a long-shot bet by the name of "Lucky Day". Such a million dollar bet is noticed nearby by a beautiful lionfish named Lola, who flagrantly seduces an excited Oscar, but Oscar is disappointed when she leaves upon learning that he is a whale washer. Sykes is furious that Oscar bet the money but nonetheless agrees to see how the race turns out. Moments before their "horse" "Lucky Day" crosses the finish line, he trips and falls on the line. The race is lost and Oscar is set to be punished in a secluded area for his impulsiveness.
Meanwhile on another side of the ocean in the wreck of the RMS Titanic, a family of criminally-inclined great white sharks has a problem with one of their sons Lenny who is a vegetarian. Lenny refuses to act the part of a killer and wishes to not have to live up to those expectations. His crime lord father Don Lino orders Lenny's more savage big brother Frankie to tutor Lenny in the family business. After the two sharks depart their father, Frankie sees Oscar being electrocuted by Sykes' two jellyfish enforcers Ernie and Bernie and sends Lenny off to attack. The jellyfish spot Lenny and flee leaving Oscar alone with him. Instead of attacking Oscar, Lenny frees him upsetting Frankie who becomes annoyed and charges at Oscar. Frankie is killed however when an anchor falls on him. Lenny flees, overcome with grief and guilt. As no other witnesses were present and Oscar was seen near the body, everyone comes to believe that he killed Frankie, an opportunity that Oscar decides to exploit for fame.
Oscar returns to the city with a new title of "Sharkslayer". Sykes becomes his manager, Lola becomes his girlfriend, and Oscar moves to the "top of the reef" to live in luxury. At the same time, Don Lino has everyone out looking for Lenny. When several sharks get close to Oscar's neighborhood, Oscar's neighbors expect him to drive them away. When Oscar runs into Lenny, Lenny (who does not wish to return home) forces Oscar to let him stay with him since he is aware of Oscar's lie. Soon, Angie finds out about the lie and threatens to tell everyone. Oscar and Lenny convince her to keep quiet, though she is heartbroken by Oscar's dishonesty. Oscar's situation is not helped by the shallow Lola, who indicates to him that her interest in him extends only as far as he remains famous. With Don Lino planning revenge, Oscar and Lenny stage an event in which Lenny pretends to terrorize the town and Oscar must defeat him throwing him into the depths of the ocean. Though this further cements Oscar as 'Sharkslayer", it greatly angers Don Lino. Oscar leaves Lola for Angie after Angie reveals that she had feelings for Oscar even before he became famous, but this leaves the rebuffed Lola determined to get revenge.
Oscar buys some Valentine's Day gifts for Angie, but before he can present them to her, he finds that Don Lino has kidnapped Angie to force a sit-down. Lenny comes along now disguised as a dolphin named Sebastian. They arrive at the meeting to find Lola next to Don Lino, while Angie is tied up and gagged and presented to Don Lino on a plate who prepares to eat her if Oscar does not comply. Lenny grabs Angie into his mouth, but later regurgitates her. When Don Lino realizes that "Sebastian" is really Lenny, he chases Oscar through the reef. Oscar heads for the Whale Wash and ends up trapping both sharks. He is given an ovation by the other fish, but Oscar confesses that he is not a "Sharkslayer" and that it was a falling anchor that killed Frankie. He then tells Don Lino that everyone likes Lenny for who he is strongly urging him not to prejudge people before he knows them properly and to not make the mistake he made in prejudging his wealth. Realizing that Oscar is right, Don Lino apologizes to Lenny and reconciles with him while making peace with Oscar, stating that he and his gang bear him no ill will. Oscar forsakes all the wealth he has acquired, makes peace with the sharks, becomes co-manager of the Whale Wash (now frequented by sharks, killer whales, and swordfish), and starts dating Angie and enjoys a happy, honest life.
During the credits, Lola trying to find Oscar in the "top of the reef" wanting to make amends with him for what she did. All she finds is a hermit crab named Crazy Joe, one of Oscar's friends, waiting for her.
- Oscar is an extremely unlikable main protagonist.
- Poorly-written humor (as well as some stronger humor). This results mostly weak humor in the movie such as horrible fish related puns.
- The character designs are hideous, especially Oscar, who has uncanny resemblance to his voice actor Will Smith.
- On that topic, the fact that the faces of the characters resemble their voice actors was distressing for some viewers, and it was even considered somewhat grotesque.
- The story is very predictable as it follows the "liar revealed" cliché.
- It can't seem to decide whether or not it's for kids, as it constantly vacillates between juvenile humor and pop-culture references that only adults would get. Not to mention an ending that tries to have a very adult moral about tolerance, but got bogged down by the constant juvenile humor, therefore not treating the issue with any measure of dignity.
- Sub-par voice acting. The use of celebrity voices was pointless and only for money.
- References from R-rated gangster movies such as "Goodfellas" and "The Godfather" in a kids movie.
- In one of the scenes, Sykes tries to kill Oscar because he lost the bet (In a kids film) and the film expects us to believe that we should feel bad for Angie, since Lola kissed Oscar.
- The underwater setting is almost pointless, and was just made to cash in on Finding Nemo.
- The animation is ugly, but looks well-made.
- Its original soundtrack is a combination of rock, hip-hop and real pop in the late 20th Century performed by very talented, catchy musicians.
- Robert De Niro, Martin Scorsese, Ziggy Marley, Doug E. Doug and Jack Black do a pretty good job as guest voice actors.
- Oscar/Will Smith is pretty good as a singer.
- Frankie’s death was very touching.
- The concept of an underwater city is somewhat interesting, despite being almost pointless.
- Some funny moments here and there.
- Lenny is actually the most likable character in the movie and is actually pretty funny.
- Jack Black and Angelina Jolie later starred together in the Kung Fu Panda franchise as their characters (Po and Tigress).