You can't really compare nightlife here to that of Philadelphia. If you're much into the club/dance scene then Ybor city in Tampa on the weekends is hopping with action. Also, look into bars/restaurants in the Soho/Hyde Park area. Downtown St. Pete has many great restaurants, many bars, ice cream shops, concert venues, visual/perfoming arts center, and a movie theatre all within walking distance from one another so obviously this is a pretty happening place on weekends, and to a much lesser extent on weekday evenings.
For seafood, I don't know your price range but check out Mid Peninsula Seafood on 49th street. Cheap and simple. Park Shore Grille in downtown St. Pete is a great new place. They serve several seafood entrees like grouper cakes and barbeque shrimp. Also check out Crazy Conch Cafe out on Tierra Verde if you get a chance. Oh, and Keegan’s Grill on Indian Rocks beach. That should keep you busy for a while.
For sushi, I like Sushi Fune in Largo.
Mexican? El Toro Negro in Kenneth City (next to St. Pete), Taco Nazo, and Mi Mexico in Tampa.