Boston Fish Pier - Retail?
It's not really a retail area - Sea to You used to be a great spot for sashimi quality fish, but it's been downhill for a couple of years even if it's open.
The nearest good options would be Yankee Lobster deeper into the Seaport district but not too far to walk, and J. Hook Lobster right on the edge of downtown - they have pretty good to excellent other offerings in addition to the very popular Lobsters - and their takeout lobster roll is tasty, fresh lobster, but it is unfortunately pre-assembled and ends up on a refrigerated white bread roll (vs. delicious butter-grilled and warm white bread roll) which ruins it for me.
300 Northern Ave, Boston, MA
Not as big a selection as I would have thought like Ming's but enough to find something you would want. Tilapia, shrimp, clams, mussels, etc. I didn't see lobster but it could have been out back.
Also, some of the fish was frozen. The guy said if they don't sell it on the first day, they freeze it. The frozen fish were whole and more unusual types of fish. He told me that what they had frozen would be sold by the end of day.
They also sold chowder but it was frozen in both small and gigantic bags.