Question

what is the reason of propulsion in fast swimming fishes​

Answer 1

the reason of propulsion in fast swimming fishes is the Newton's third law of motion

Answer 2

Squid swim more slowly than fish, but use more power to generate their speed. The loss in efficiency is due to the amount of water the squid can accelerate out of its mantle cavity. Jellyfish use a one-way water cavity design which generates a phase of continuous cycles of jet-propulsion followed by a rest phase.