One of my favorite things about eating in Istanbul is the preponderance of cheap, awesome seafood. (Apparently, mid-November is a great time for fish-eating. I am happy to time all of my vacations around fish-eating from now on.) And the easiest way to eat said fish is from a street stand—just about every one selling balık ekmek (translating literally to "fish bread": a simple fish sandwich.)
On both sides of the Galata Bridge, in the waterfront neighborhoods of Eminonu and Karakoy, you'll see tiny grills set up with bold BALIK EKMEK signs. (It's a little disconcerting that these stalls sit so close to the hundreds of amateur fishermen casting their lines off the bridge, into what must surely be terribly polluted waters. But the street vendors won't be serving those fish.)
Here's one from the Karakoy side of the bridge, at a little waterfront restaurant affiliated with a fish seller adjacent to it. (This one was 5 lira, about $2.75 USD; most around the area seemed to be 4 or 5 TL.) The sandwiches are simple: a mackerel fillet quickly grilled, salted and sprinkled with red pepper, served with onion, lettuce, and a wedge of lemon for squeezing over the top. Nothing fancy, but simple, fresh, and totally satisfying.
Kariköy Fish Market
At the end of the Kariköy Fish Market to the west of the Galata Bridge