They are not fish.

Whales and dolphins belong to the order Cetacea among mammals. Seals belong to the order Pinnipeds among mammals. Turtles are reptiles. You ask why they are not fish. The simplest and most fundamental difference is that fish breathe through gills, while mammals and reptiles have evolved to breathe through lungs. Fish can use their gills to breathe oxygen in the water, while mammals and reptiles must expose their breathing space to the air to breathe.

The morphology is indeed similar to fish. For example, after being active in the water for a long time, natural selection seems to have equipped them with flippers suitable for swimming, which greatly increases the efficiency of swimming. For the same turtles, you can tell whether they live in water or on land just by looking at the shape of their limbs.

The ones most similar to fish should be whales. Long-term life in the water has made their bodies streamlined. Their forelimbs have evolved into fins, their hind limbs have disappeared, and their tails have evolved into caudal fins without bone support. They are different from whales. The simplest method for animals and fish is that whales have no gills, their tails are horizontal, and they rely on up and down swings to provide forward momentum. Fish have gills and a vertical tail that moves forward by swinging from side to side. (Except for a few that have evolved to have a flat body, such as flounder, there are also those that do not need to swim, such as seahorses, whose tail has degenerated into a propulsive function.)

Fish have several major characteristics: 1. Have gills 2. Fish egg reproduction 3. Unable to live on land

Whales: They are viviparous, and milk is used to support young whales. Is a mammal

Dolphin: Same as above

Sea turtle: lays eggs, can be amphibious on land and water, has no fins. (Do you think he looks like a fish?)

Seal: Same as Xiaoli, there is no need to ask questions, it should be search~~Hehe O(∩_∩)O~ However, I copied this Answer...