I have had sushi that was completely fresh — literally a tuna I caught myself, carried off the boat, cut up and ate. The taste and texture was fantastic.

You are right that almost all fish you eat has been frozen. This is partially for practical reasons: fishing boats are often at sea for weeks at a time. Also for safety reasons: most fish have a significant risk of parasites which are killed by deep freezing.

Tuna is one of the few fish safe enough to eat fresh and raw (and even then there is some amount of risk).