Shore fish restaurants at sunset...
Karaköy Fish Market
The chef toils over an open griddle or grill frying fish, whole or filleted, sliding them onto plates with some salad and handing them to a waiter who serves you at a low table-and-stool. Fresh bread, soft drinks (no alcohol), low prices: less than TL20 for a fish lunch or dinner.
One caution: sometimes the chef, anticipating a rush of customers, will fry up lots of fillets well in advance, then heat and serve these pre-cooked fillets when customers arrive. These pre-cooked fillets are not as tasty or tender as freshly-cooked fillets. Look at the grill, and if there's a pile of pre-cooked fillets, perhaps you want to choose a different restaurant.
Kumkapı Fish Market
No doubt you have heard about Istanbul's historic Kumkapı district, cluttered with (fairly expensive) fish restaurants.
It used to be that if you passed through Kumkapı and crossed the busy shore boulevard (Kennedy Caddesi) to the waterfront you'd see the Kumkapı Fish Market (Balık Pazarı).
However, this area is now a construction site for roadwork. The fishmongers have been moved to Samatya, at least temporarily (probably meaning "for years.")
The Kumkapı fish sandwiches were superior to the more famous Balık-Ekmek served from the fancy boats by the Galata Bridge, but for the time being, you'll probably want to choose Galata Bridge instead. More...
—by Tom Brosnahan