Theory:

Locomotion in fishes:
Fishes are vertebrates with an endoskeleton made up of cartilage or bones.
The fishes have a streamlined body where the head and tail are small when compared to the mid-portion of the body. This shape of the body is known as streamlined.
The fishes have a streamlined shape that allows the flow of water around them quickly (by reducing the friction of water), thereby moving the fishes in water fast.
 
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The various other characteristics that are responsible for their movement in water include the presence of a skeleton which is covered by strong muscles. Fishes swim in water with the help of fins. They have two paired fins and an unpaired fin.
When a fish swims, these muscles make the front part of the body of fish to curve to one side as shown in picture, and the tail to move in the opposite side and vice versa. It pushes the fish to move forward by causing a jerk.
 
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                   Movement in fishes
 
These jerks occur in series with the help of the fins of the tail making the fishes to move ahead.
The other fins on the body of the fishes maintain the balance of their body so that they keep the same direction while swimming.
Locomotion in snakes:
The snakes are vertebrates in which the vertebral column (backbone) is very long. Their body is made up of many thin muscles which are far from each other. Inspite of this their muscles are interconnected to each other, the backbone, ribs and the skin.
 
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          Slithering movement of snakes
Important!
Unlike other animals, snakes lack limbs. Instead, their body is covered with hard scales that help them in crawling.
The body of the snakes form loops when it curves. These loops push the snake forward by pressing against the ground. As the snakes have long bodies, they make many loops. Each of these loops pushes the snake forward, thereby making them move very fast but not in a straight line.
The movement of snake is known as slithering movement. Most of the snakes can swim in water also.
Reference:
https://www.aplustopper.com/movement-of-fish-in-water/
https://pixabay.com/photos/aqua-aquarium-fish-seabed-algae-5172022/
https://www.shutterstock.com/image-photo/moorish-viper-macrovipera-mauritanica-poisonous-white-97140149