Rio de Janeiro is famous for its beaches, bathed by the Atlantic Ocean. It should come as no surprise that there are outstanding seafood restaurants in the city. If you are not budget conscious, Margutta or Satyricon are the first ones you should try. The excellent seafood is served with an Italian accent, the wine lists are worth going through. These are sophisticated restaurants in Ipanema. Associate the meal to a visit to the jewelers and boutiques on Rua Garcia D'Ávila, and surroundings.
Marius Crustáceos, in Leme, has an interesting proposal. There is a self-serve buffet with oysters, squid, sea urchin, tuna, salmon. Hot dishes like seafood spaghetti are also included in the fixed price. Don't even try to pronounce the name, a tongue twister unless you speak Italian or Spanish. Not far is Shirley's, one of the most traditional restaurants in Leme. They serve seafood prepared Spanish style in a cozy atmosphere.
Marisqueira, in Copacabana, has a tradition of 50 years, and a Portuguese accent. Sol e Mar is another Portuguese option. Still in Copa, Pampa Oceano serves fresh seafood rodizio-style (all you can eat) and the hot dishes are excellent. At Siri Mole & Cia the accent is Brazilian. Azul Marinho in addition to seafood features a lovely view to Arpoador Beach.
You will probably spend one day beach-hopping, and a perfect way to end it is with a meal at Candido's, or Tia Palmira, off-Barra. If you went by taxi it would cost you a fortune. This is the kind of adventure that is more fun to do together with a local who knows how to get there (and owns a car). The food is worth it, and portions are big enough for two. For a romantic date choose Laguna, the meal starts with a boat trip to the island where it is located.
In the Downtown area try Albamar. It's a good choice to associate to a walking tour around the historical attractions, churches and museums nearby. Sentaí is open for lunch only, serving the best lobster by far. At rodízio style restaurants they also serve barbecued fish steaks, usually salmon, and Brazilian fish like the robalo, or congro. Of course you will find the mandatory assortment of sushis an sashimis at Japanese restaurants.