Fresh seafood should never smell like anything else but the ocean. If it is stinky or has a very powerful odour, then don't buy it or eat it. Better to be safe than sorry than taking on the risk of foodborne illness. The eyes of your crustaceans/shellfish should be bright and black. The seafood shouldn't be damaged or mangled. Make sure that there isn't any black or discolouration to the cooked prawns/shrimp. The shells of any live mussels clams should be tightly shut. Or close when you gently tap them. If they don't close when tapped it could indicate dead mussels or clams.