Tidus didn't really disappear
At the end of FFX, everyone believes that Tidus was a dream of the fayth and that he disappeared and went to the Farplane, because Sin was defeated. But that isn't the truth.
See, what happened was the Fayth had a giant party, and all got really drunk. They thought it would be funny to play a prank on Tidus and tell him that he was a dream of them, and that he would disappear once Sin was defeated. Anyway, that was a big load of crap and when Sin was defeated the Fayth were all very unconcious because of their earlier party.
Anyway, at the end Tidus was ready to go to the Farplane, but Yuna was being persistent, so he casted Haste on himself.
This allowd him to move at the speed of light and he was able to move out of the way and back again as Yuna tried to hug him, while making it look like he hadn't moved.
But then Yuna fell over, and he felt sorry for her so he hugged her.
Eventually, Tidus jumped off of the Airship to reach the Farplane. It is at this point that everyone believed that the Fayth stopped dreaming, and he was sent to the Farplane.
But what happened is, a big Seagull grabbed him as he was falling, and took him to his nest. On the way he fainted and believed he was in the Farplane, high-fiving Jecht.
The Seagull attempted to eat him, but spat him back out at the taste of his Max Strength hair gel.
He then landed in the oceans of Spira, where he believed he had been reincarnated as some sort of Fish (There are no mirrors underwater). So he lived with the Fish for 2 years, adapting to the "Fish Lifestyle".
1 day, 2 years later he was swimming along, looking for food when he almost got crushed by a giant Airship. He bobbed his head up for a quick look (Only a quick look, Fish have gils after all) to see a much hotter Yuna and Rikku, and their equally hot friend. He flapped towards them, only to realise he had legs, and was human.
And the rest as they say, is history.
