Well realizing that he was about to be obliterated by Lavos, somehow i don't think being vaporized or crashing into the ocean and dying sound anything like becoming immortal. I kinda think that he maybe realized that he was gonna die, and maybe, just maybe, he decided to try and save himself, unless he thought that being disintegrated or drowning meant being immortal, i don't know, i'm not an expert on immortality, but i'll tell what he didn't do, become IMMORTAL!