All I ever find are seaweeds, spiny lobsters, anamone, or nothing because the shadow gets away. Is there a trick to find Scallops, those things that Pascal needs? I also need the for the Harvest Festival.
I got one during the wetsuit trick, but my pockets were full of flowers because of it so I couldn't keep the damn thing...X_X
Anyway, if you find a large shadow that moves away, swim on the surface until you are directly over it and then dive. The shadow will gain some distance, but should be close enough still that you can catch up shortly before your resurface. At least that's how I catch all the fast things.
I never find them regularly. One tip to diving to catch something on the move is be prepared to head opposite direction. They almost always turn the other way once you dive.
They move quickly and radically. Try to be close to the ocean boarder so you can block them from crossing over. Then dive and rapidly swim underwater towards them.