Everyone has heard of the Japanese fish market, or the one in Busan. However, there's a great fish market in Seoul!
It's located in Noryangjin Station (Line 1) exit # 2, and it is relatively close to the subway station so you won't have problems finding it.
The best fish will come around 5am-6am where all the fishers bring in the good stuff. You could also go to dinner time and the market and restaurants will be filled with people fighting for the best fish : )
My friend and I got a giant Flatfish fo 40,000W which is definitely a great deal in Korea. However, it's all about the negotiation skills!
Usually, people take the fish (cleaned and cut) to the restaurant to eat it and get the full fish market experience :D
I will definitely be going back sometime soon--will try to upload pictures of different fishes !