Hmmm... it's been a couple of years since I was in the Cinque Terre, but I definitely had some good seafood in Vernazza down by their small harbor. Can't remember the name of the restaurant, though.
Truthfully, though, I usually eat in the restaurants north of the Cinque Terra whenever I'm in the area - in Chiavari and Sestri Levante specifically - as, in my opinion, the seafood is better and less expensive.
There are no "fantastic" restaurants in the Cinque Terre. As S B Russell said, the "better" restaurants are north of the Cinque Terre, in Chiavari, Sestri Levante, near Camogli etc.
Our favorite restaurant in the Cinque Torre,, by far is Cappun Magru in Riomaggiore. Not fantastic but very good. Not all seafood, but half the menu. Good wine list. Fantastic location. Polpo... perhaps but not every night.
In the Cinque Terre one finds a number of good seafood trattorie. Nothing special but excellent fresh fish. Stick to the simple preparations because the trattorie are not capable of more. We like Gambero Rosso in Vernazza.
Piazza Marconi 7, Vernazza, Liguria , IT
Sorry I didn't see this post in time for the original poster, but for others who might dig it up later:
Polpo Mario in Sestri Levante, just a quick train ride from the le Cinque Terre area, has a marvelous and extensive octopus menu. The plain steamed octopus, drizzled with olive oil, is my favorite.