Modern fishing boats often spend more than a week away from the port. Therefore most fish has been frozen before it is coming on land for the trade. Flash freezing ensures that the fish quality remains well preserved. Otherwise you best buy fish alive or fresh fish that is kept on ice. Fattier fish spoils faster than lean fish and deep ocean fish goes bad quicker than tropical sea fish. This is because fattier fish contain more oils that can oxidize. And fish that live in the colder, deep sea have all their enzymes optimally work at lower temperatures as well. Thus they will spoil quicker, even at low temperatures, unless gutted and placed on ice.
In Hong Kong you find live fish at the market. Always ask to add some ice, after it is killed and cleaned. Also crustaceans such as shrimp or cray fish have their guts in the head. If the shrimp is dead and as long as the head is on, the decay process will continue, even on ice and even when frozen (albeit slower).
Defrosting is best done in the refrigerator overnight or in cold water for 30 min (water can be refreshed if required). Always pat the fish dry before further using to cook.
Practice caution when consuming fresh fish or shellfish raw: In general a freezing step is recommended to kill off any parasites. Live oysters always have the risk of a virus contamination (most commonly the norovirus), unless they are cooked. Go for oysters from well-established regions. Similarly, ensure mussels are properly cooked (but not over-cooked) for the same reason.