Well, underwater, most animals don’t stop moving, so I imagine they don’t put the same stresses on their joints and muscles that land based animals do. That would eliminate the need for stretching, IMO. For example, if I sit at my computer for long hours every day, it put stress on my lower back, and other muscles. I need to stretch to alleviate these pressures. Similarly, gravity and body weight puts stresses on the joints and muscles of land animals. But underwater, this stress is lessened. Since fish keep moving, and don’t really sit or lay down, it would seem (to me at least) that their body weight wouldn’t factor in as a stress to their muscles and joints. This in turn, may alleviate the need for stretching.
Again, this is just off the top of my head, and I could be wrong.
Perhaps @Marina is right, and I’m confusing the ability to stretch with the necessity of stretching.