The Nile is not only a beautiful river, but a river full of life. In local markets you will find many species of fish. Most probably you will not have time to cook them. I recommend you to taste them in one of the many fish restaurants in Aswan. Most of them are located by the Nile, in Corniche. Some of these restaurants are on big boats or on platforms on the river.
The most famous restaurant in town is the "1902 restaurant", in the old cataract hotel. If you are looking for a cheaper place to eat, you will find many options in Sharia as-Souq (bazaar) and by the train station.