This fish market is as much a sightseeing spot as a place to eat. The speciality here is called katte-don (勝手丼) – literally 'rice bowl as you like it'. First buy a bowl of rice from one of the vendors on the perimeter then head to a fish monger and have them top it off with your choice of raw fish. If you want to get even more in the mood, rent a kimono (¥1000) and walk around in style.
It's a couple of minutes' walk south of JR Kushiro Station.