I hate both of them.
Finding Nemo is Pixar's worst film. Compared to classics like Toy Story and Monster's Inc. Finding Nemo falls incredibly short on the entertainment factor. The storyline is incredibly predictable and quite frankly boring in places. The dialogue is full of too many cliches, and whilst it's still funny... I just don't like it XD
Shark Tale only got mass cinema attendence because of the reel of celebrities that voiced characters, and that Dreamworks produced Shrek, so people automatically hink it will ultimately be up to Shrek's quality. In truth, the storyline is worse than Finding Nemo, and it isn't even that funny.
Both are disappointing, but if I had to pick a worse one... probably Shark Tale.