When Alister dived into the Sea Of Chaos, he didn't use any of his energy to protect his body.
Before, he could somewhat survive the chaotic waters of the Sea Of Chaos due to covering his body with his mana and other energies. Even when he became the strongest, Alister dared not come into contact with the chaotic waters or else it would destroy his body.
But after obtaining the Chaos Body, not only did his affinity towards the chaos element and chaos law improve, but Alister also became more resistant to the chaos element.
When his body was submerged in the Sea Of Chaos, Alister's clothes started disintegrating until all of his clothes were destroyed and he became naked.
Afterward, the chaotic waters in the Sea Of Chaos assaulted his body!
As if his body was torn apart and pulled and compressed against each other at the same time, the chaotic waters played with his body as if he were a lump of clay.