In the book "The Once and Future King," what was the lesson the Wart learned as a fish?

When Wart is transformed into a fish, he is taught most immediately what it is to learn for the first time the basic rules of locomotion, and must discover by observation how to propel himself through the water.  He must know to heed even the slightest detail in his own movement and that of Merlin, and he learns that all creatures are constructed differently and adapted to their own environment.

Wart learns about this new...

When Wart is transformed into a fish, he is taught most immediately what it is to learn for the first time the basic rules of locomotion, and must discover by observation how to propel himself through the water.  He must know to heed even the slightest detail in his own movement and that of Merlin, and he learns that all creatures are constructed differently and adapted to their own environment.

Wart learns about this new environment, and the dangers and secrets of the natural world surrounding him; he is given a new perspective on all the things he thinks he understands innately as a human boy, but which he in truth has only an incomplete comprehension of.  By seeing the moat in microcosm, in absolute detail as one of its own inhabitants, his understanding of the universal system in which he, as a boy, exists, is expanded.

At the end of his foray into the underwater realm, he meets with the King of the Moat, a giant four-foot pike with tyrannical tendencies, who introduces to the Wart the concept of Might is Right, which is an integral theme that King Arthur must contend with throughout the book.  The pike contests that this philosophy is absolute, and that there is nothing greater than bodily power: “…power to grind and power to digest, power to seek and power to find, power to await and power to claim, all power and pitilessness….”  At last, in a very symbolic gesture, the “disillusioned” pike attempts to eat the Wart, who must save himself by using his newfound skills as a fish to swim away as quickly as he can.  This is a representation of the struggles Arthur must face in his own rule as he strives to reign justly and weigh against his actions again and again the idea of Might is Right.