The Eastern Market Seafood Company is located nearby Hirt and Co and the Cost Plus Wine store in Eastern Market. If you're looking for a great deal on fish, I would go across the freeway to the Gratiot Central Market and get fish over there. If you're looking for a large variety and/or more unusual stuff (like octopus, etc) this is the place to go.
Also, on Saturday's they fry stuff for you to eat immediately. I always walk by and smell the delicious food on my way between the sheds and Gratiot Central Market.