Tidus is officially pronounced Ti-daa due to the following fact:

Quote Originally Posted by Wikipedia
Nomura has expressed after designing serious and moody main characters for Final Fantasy VII and Final Fantasy VIII, he wanted to give Tidus a cheerful attitude and appearance, which is reflected in the name Kazushige Nojima chose for him (Tidaa is the Okinawan word for "Sun").
Consider that Tidus' sigil and whatnot are paired with the sun through out the game. Knowing this, you can't deny it.

HOWEVER

This is now somewhat broken due to the coming of Kingdom Hearts 2...

Quote Originally Posted by Wikipedia
Because his name is never spoken out loud in Final Fantasy X, its intended pronunciation has been a subject of debate among fans. Interviews with James Arnold Taylor, Tidus' English voice actor,[7] and spoken dialogue from the English version of Kingdom Hearts—which featured the character in a cameo—portray it as "tee-dus" (IPA: /tiːdəs/) whereas one instance in the English version of Kingdom Hearts 2 exists in which the character's name is uttered "tie-dus" (IPA: /taɪdəs/).
So what was official is now less official, if that makes any sort of sense.

Personally, I call him Tie-dus because I think Ti-daa sounds smurfing retarded.

Tidus - Wikipedia, the free encyclopedia