Siri Mole & Cia. mentioned above has seafood, but Bahia style. In Rio you can also find very good Portuguese restaurants serving codfish!
The best sea food restaurant in Rio is Satyricon (book in advance), it is just accross Capricciosa (Pizza) and near Porcao Ipanema (churrascaria), price range between 60 and 120 local currency. Satyricon also has a branch in resort beach city of Buzios.
Satyricon
http://www.satyricon.com.br/
Rua Barão da Torre, 192 (Ipanema)
Margutta
http://www.margutta.com.br/
Rua Henrique Dumont, 62 (Ipanema)
Ipanema
Marius Crustaceos (price range above 100 local currency but big servings)
www.marius.com.br
Avenida Atlântica, 290 (Copacabana, almost in Leme)
Don Camillo (more simple, life music, next to Copa Cafe, in front of Copacabana beach)
www.doncamillo.com.br
Avenida Atlântica, 3056 (Copacabana)
Rgs,