I'd like to start with an analogy. Look at what you said in comments that some vegetables are actually better purchased frozen than purchased fresh. Let's take frozen peas vs fresh.
Peas notoriously begin to lose flavor and their great texture as soon as they are picked. Quality manufacturers of frozen peas get those peas frozen at extremely low temperatures almost immediately after they are picked. From the time they are frozen until the time you use them, typically they have stayed frozen.
"Peas go into the freezer at one end at about 15C, and they come out six minutes later at the other end at around -18C."
The peas make it into the freezer to be flash-frozen in the two-and-a-half hour time limit.
So, if you can buy fresh peas that were picked no more than 2 1/2 hours ago, they might be a tiny bit superior than peas you can buy frozen. Otherwise, you should buy your peas frozen. Frozen peas are "fresher" plus they haven't been banged around and damaged in transit.
Look at seafood that way
Can you get your hands on it within a few hours of it being caught? If not, purchase it frozen. Leave the freezing to the people who have the equipment to do it well, and will do it fast, sometimes while still on the boat.
In the US, most seafood arrives at the grocery store frozen. Then they thaw it out, put it on ice, and display it more attractively than it looks bagged in the freezer. It makes no sense to buy that. Buy exactly the same fish while it is still frozen, because it starts to lose freshness again as soon as you start to thaw it.
This basic rule is true of every variety of fish I know of plus shrimp, squid, scallops, lobster and octopus:
Buy it nearly off the boat, buy it live, or buy it frozen
"There really is no difference," said Gibbons. "The clock never moves backward when it comes to freshness. If a fish is caught, handled well and frozen immediately, you literally stop the clock. You freeze in the freshness." He adds that nutritionally, nothing is lost when fish is frozen.
These days, technology is such that fish are either frozen right at sea (most common with farmed fish, as freezers are incorporated into the farm sites) or immediately upon landing at port, said David Pilat, global seafood buyer for Whole Foods Market.
And the assumption that fattier varieties such as salmon and tuna fare better, texturally speaking, than leaner fish when frozen doesn't hold true, either. Our experts say it comes down to proper freezing and handling on the front end, and proper thawing—in the fridge, out of the package—on the back end.
"There is no downside to buying frozen fish," Gibbons said.
If your store displays thawed fish on ice, look at it first and then the frozen package of the same item:
Commercially frozen fish is quickly frozen at its peak freshness. Consumers can now find a wide choice of top-quality and wholesome seafood in the freezer case. When properly thawed, frozen fish is comparable to fish that was never frozen. Both exhibit the qualities of freshness described previously. Frozen fish and shellfish should be packaged in a close-fitting, moisture-proof package. Select packages from below the load line of the freezer case. Look for packages that still have their original shape and the wrapping intact with little or no visible ice. Seafood should be frozen solid with no signs of freezer burn, such as discoloration or drying on the surface, and have no objectionable odor. The same guidelines apply for frozen prepared seafood, such as crab cakes, breaded shrimp, or fish sticks. Do not allow the package to defrost during transportation. When properly thawed, frozen fish can be comparable to fish that was never frozen.