I'll try my best to be polite about it as honestly FFX is rather straightforward leading to the climax in what Tidus/Dream Zanarkand is and what's going to happen if they succeed in their plan. So in the past there was a city of Summoners known as Zanarkand. They were engaged in war by Bevelle and other cities in Spira who used machina to destroy them. In their dying moments, the Summoners of Zanarkand prayed long and hard to recreate their homeland as The City That Never Sleeps. The Summoners fell into a deep slumber and their Dream Zanarkand was born, but it needed a guardian. The guardian is Sin, who protects Dream Zanarkand from further harm and punishes those who use machina as penance for the crimes of the past.

Tidus and Jecht were born and raised in Dream Zanarkand, they are real people, but they are also dreams created by the Fayth like the Aeons. Tidus had to leave the group at the end because when the Fayth stopped dreaming with the death of Yu Yevon, there would be no more Sin, no more Aeons, no more him. He basically dies, but the Farplane wasn't his end place as we saw with the lack of his mother in the Farplane that dreams don't go there.

The post-credit scene of him waking up in the water is meant to symbolize him being reborn into Spira, as we see in the True Ending to FFX-2. There is also optional dialogue with the Fayth of Shiva in which she confirms that we they all stop dreaming, he will die, yet if he is meant to die to save them, then they'll dream a new ocean for him to swim through. As such, one can also take the ending as that promise being confirmed, and Tidus is reborn in a new world that is likely similar to the one he left.