The most streamlined sea animal is often considered to be the dolphin. Its fusiform body shape reduces drag while swimming, allowing it to glide smoothly through water. This adaptation not only enhances its speed but also improves its efficiency in hunting and escaping predators. Other streamlined animals include sharks, which also exhibit similar hydrodynamic features.
Friction.
Animals like sea urchins, some species of pufferfish, and certain types of crabs are not streamlined. Their body shapes are more round or bulky, which can make them slower swimmers compared to streamlined animals like dolphins or sharks.
sea urchin
It's shape.
Whale
an animal, car or boat are streamlined because most animals, cars or boats have round corners and they are low to the groung this means that they go faster because streamlining helps us go faster and the rounder a shape is the faster it will go
octopus and star fish
Yes, if you're comparing it to a buffalo.
killer whale
...
the catfish
the most ancient sea mammal would have to be the flat fish although it is endangered.