There is little information telling us exactly how the Kokiri die if they leave the forest. As many people have already stated, it is likely that monsters will kill them. There is also another theory that they just start aging if they leave the forest.
just because the kokiri never age doesnt mean they never die. i would assume that they still die after they reach a certain age (although they dont look that age), but you do have a good theory here. i believe that if they leave the forest its more likely that monsters kill them than it is likely that they just age to death really fast.
Don't know about Zoras, never heard that in the game, as far as I can remember anyway. But even if it is true, it could simply be that they only left the water for a short time to party.
the Zora can leave the water. in the Zora domain in OoT there are plenty of them that are not in the water but just standing around it. also, when Link turns into a Zora in MM he can walk around on land as long as he wants. i hope this answers questions on whether Zora can leave water or not.
As for Rauru, he was always trapped in the Sacred Realm. He never left it once, except when he appeared in Ganon's Castle (and I believe that he wasn't actually there, I think it was just some image of him that he created.) As for him being Kaepora Gaebora... I highly doubt it. The Gossip Stone stated that the owl was a reincarnation of an ANCIENT sage. Rauru doesn't strike me as ancient.
i also dont believe the owl is Rauru. that would have been explained in the game somehow. as for how ancient Rauru is, we dont know how old is he is or if he even ages as a sage so he could very well be ancient but i still dont believe he is the owl.