Everyone takes Bosphorus ferries but the Golden Horn route is way more interesting. Starts at Eyüp (near the mosque complex), stops at Fener/Balat, then Karaköy. 45 minutes, 25 TRY, runs every 30 minutes.

You see the old Byzantine seawalls, colorful neighborhoods from the water, and finish at Karaköy's modern art scene. Much better perspective than walking those steep Balat streets.