Unlike mammals and birds the muscle structure of fish is made of rows of short fibres separated by fragile sheets of thin connective tissue. This shifts the cooking focus from tenderising (as with land animals) to one of simply firming up the muscle and melting away the connective tissue.
Visualise the typical piece of fish to cook as a row of small balloons filled with a protein substance, with each balloon of protein held to the next by a thick glue. When the row of protein balloons are heated, the inside firms up and changes color. The glue holding the balloons together simply melts away. The fish is then at its most tender, moist and flavorful. Cooking further will now only dry out and toughen the flesh.