Here's the rough physics of the situation. Swimming speed is basically a function of the force applied to create thrust forward and the drag created by friction with the water.

Think about the shape of a torpedo or football. This is called a fusiform shape and it allows water to flow smoothly over the object, causing very little drag. Any time that something sticks out to the side, it disrupts that smooth flow and creates more drag. A rudder is a controlled use of drag.

An eel swims by moving the rear 2/3 (roughly) of its body. This means that a lot of its body is sticking out into the flow most of the time, creating drag. At low speeds, this is no big deal. However, this lack of good hydrodynamics prevents very fast swimming.

Something like a trout uses about the last 1/3 of its body to move back and forth, with its caudal (tail) fin providing a lot of the thrust. Less of its body is being thrust sideways into to the water flow, creating less drag.

A tuna keeps its body very stiff and just moves its tail, greatly reducing the drag. A large stiff tail with good muscles helps to increase their speed.

Think about a modern boat. A section of the boat through or below the waterline has a fusiform shape. The propeller is small and placed at the rear. Question for your class: Why not have a fish tail rotate like a propeller?