Don't bother seeing Shark Tale. Instead, watch half of Finding Nemo and chase that with a half-hour of listening to R&B. You'll undergo an equally unsatisfying experience. Whereas Nemo is simple and appealing, Shark Tale tries actively to be hip and edgy, but should have made the entertainment aspect a higher priority.
The film has two basic plotlines. The first is centered around a nobody fish who wants to be a somebody fish and attempts to accomplish this through lying and deceit. The second is the "parents just don't understand' plotline.
Jack Black plays a vegetarian shark who wants to pretend that he is dead, and an overly glib Will Smith voices a fish who supposedly kills sharks. As luck would have it, the two meet, tell some lies and subsequently get into trouble. Had the dialogue been better, the viewer might actually care about the characters, but instead one only notices the celebrities who portray them.
If you're going to see this film, do so for the series of sea-themed puns on various advertising campaigns, and for one of the first coming-out-of-the-closet subplots in a children's movie -- not out of a desire to see a quality flick.