Aquatic animals have no neck

Perhaps you did not know and neither noticed, that none of aquatic animals has no neck, but directly from the head goes the body, except mammals, in which the neck is so small, that will probably disappear in the future (this applies only to animals that never come to land).
This is because aquatic animals don't need neck, because it is much simpler and more efficient to turn the whole body. Unlike terrestrial animals, aquatic animals have a much more flexible skeleton or have no skeleton, which makes them much maneuverable and faster in water.

Difference between aquatic mammals and fish

One of the main differences between fish and aquatic mammals is: fish have vertical tail, while mammals have horizontal tail. Both tail are very efficient, fish tail is a good speed and maneuvering, while the tail of mammals gives more power, which allowed mammals to grow larger than fish, such as the whales, the largest animals that ever lived.