You can either bring some fish, or buy from the fish sellers that surrounds the fish restaurant in the round shaped building.
The restaurant is called as “Samsun Büyükşehir Belediyesi Balık Pişirme Yeri”, means “Fish Cooking Restaurant”, which is operated by Samsun Metropolitan Municipality.
The price of cooking the fish is affordible. They also offer (sell) drinks and salads which you can drink or eat with your fish in the restaurant.