I feel you have been given some misinformation. Most aquatic mammals evolved from land based mammals, but the fishes have been evolving from fishes for as long

I feel you have been given some misinformation. Most aquatic mammals evolved from land based mammals, but the fishes have been evolving from fishes for as long as we have a fossil record for. The Osteichthyes or bony fished (Tuna, Salmon) evolved from Chondrichthyes cartilaginous fishes(Sharks, Rays), but none of these evolved from land based animals.