depending on what sort of place you are after, one of my favourite seafood restaurants is Toofeys in Elgin St, Carlton. i actually went last night
quite small, very classy, a little expensive, awesome food and service.
menu changes DAILY in relation to what fish is fresh that morning at market, thats how good the quality of food is.
only open tues-sat as fresh market seafood is unavailable sun & mon mornings, open for lunch and dinner.
there is another one in St Kilda too (different name, may be same owner), but apparently isn't quite as good as Toofeys.