I would guess that the size of the fish has much to do with it, so that both the freezing and the thawing can happen in a short enough time to be effectively "all at once". That way some parts of the fish don't start to decay while the rest is still frozen.
Come to think of it... It’s possible that the water in some part of the bay could become “supercooled”. If it were very still, it could drop in temperature well below the actual freezing point. Then all it would take is for the disturbance of the school of fish swirling through, and —snap— the supercooled water would solidify almost instantly.