By the first part I'm guessing you're asking about what happens to Jabu-Jabu in the Adult part of the game. Nobody really knows...Jabu-Jabu just disappears. Some people believe that Jabu-Jabu is the Windfish from Link's Awakening, they do bear a slight resemblance.

Jabu-Jabu could of just been your normal fish and been granted powers of large size by the Great Fairy living nearby...or it could of been some freak accident. Who knows?

Jabu-Jabu is a large fish and the Zoras are fish people...so why have a fish "god" while the Zoras worship a tree? That last part was sort of confusing but I needed something weird like that to make sense.