The major difference between fresh and frozen Seafood is that the frozen seafood has been on ice longer and at a lower temperature. Unless you are buying your seafood from a market or restaurant that keeps fish or lobsters swimming around in a tank for you to choose the one you want, all seafood at sometime has been put on ice to keep it fresh for the journey from its water habitat to your local market.
Most people prefer their seafood as fresh as possible. In order to get fish and other seafood that has been freshly caught you have to live within driving distance of a wharf or pier where fisherman come in daily with their catches and sell them there on the spot. Of course, if you are able, you can always go out and catch fish and crabs fairly easily yourself, but most people get their seafood from a market.
The seafood found "fresh" in refrigerated cases in grocery stores has been iced down from the moment it was caught and how fresh it actually is will depend on how far it had to travel to get to its destination. Keeping it cold slows its degeneration keeping it "fresher" but ice can only so much. To find out the true age of the seafood in your local market you will want to ask someone at the seafood counter when it came in and where it came from. Fish and seafood caught locally will be fresher though it is also possible that the seafood in your local market could have come from a nearby farm.
For those who are not so fortunate to live in areas where seafood is plentiful and affordable frozen is the next best choice since it is not as expensive as fresh. Purchasing frozen seafood can also sometimes mean a wider variety of choices as many items such as shellfish, crustaceans and mollusks are not available in all areas. However, some commercially packaged frozen seafood like all packaged foods may have preservatives added to help maintain its shelf life.
No matter how fresh your seafood is when you purchase it if you do not plan to use it right away you will still have to freeze it to avoid the possibility of it spoiling. Freezing it yourself still gives you a better chance of keeping it for a few weeks, especially if you know when it was caught. Just be sure to store it in airtight containers on which you have written the date.
Many people with discriminating palates will say fresh tastes better hands down and still others will not detect a significant difference between fresh and frozen seafood. Both are perfectly acceptable and when you can't have one the other is always a readily available option.