Make sure the skin is shiny and the flesh is vibrant. If buying a whole fish, the eyes should be clear. There should be no milky liquid on the fish and it shouldn’t have a fishy smell.
Inspect for no signs of freezer burn, ice crystals, or seafood that is clumping together. The seafood should not be discolored.
Shrimp are a bit unique in that finding a truly fresh shrimp is quite rare. Most of the “fresh” shrimp you see at the fish counter have been previously frozen and thawed. Therefore frozen shrimp actually are fresher.
Oysters, clams, mussels, or scallops:
Make sure the shells are tightly closed and they should feel heavy for their size. Again, say no to a fishy odor, but one that is briny like seaweed is good.