If you go from the fishmarket into the fruit and veg market there is a little restaurant on the left called the Oztoklu.
Rukiye ( pronounced Roo-Kee-ya ) will go with you into the fishmarket, help you buy your fish, prawns, seafood etc. You can then have it cooked at her restaurant for quite a bit less than in the fishmarket restaurants.
You'll also find that beer, spirits and wine cost a lot less than in the fishmarket. For instance, she'll charge about 10 - 12 YTL for a bottle of wine that will cost you upwards of 35 YTL in the fishmarket restaurants.