If you're staying in Roppongi, you can take the Oedo subway line straight to Tsukiji Shijo (Tsukiji fish market). The market has TONS of fresh seafood and Japanese restaurants. I personally recommend "Haibara Unagi" restaurant in the area. If you like broiled eel, this is the place to go. It is the most tender and flavorful eel you can ever get for the price (1000 yen). If you're more into sushi, then Tsukiji has lots of great sushi restaurants. If I were you, I'd just go to Tsukiji for sushi, rather than taking the sushi boat. Japanese people take the sushi boat not for sushi itself but for the boating experience.