Huge concrete building packed with market stalls selling fish.
Fresh stuff gets snapped up early (4am) in the morning by chefs, but whatever time you get there you can choose your catch and have restaurants in the same building cook it up for you.
Prices are marked so you won't get ripped off (although there's always a little room for haggling) and it's all very photogenic.
Subway Line 1- Noryangjin station (follow the map in the subway station when you arrive).