I just thought I'd add, as I don't think anyone else did, that there is an interview with the voice actor of Tidus on a bonus DVD that comes with the European release of the game. And when speaking he also says "Tee-dus". Along with Wakka in Kingdom Hearts, I guess this must be how they intended Tidus to be pronounced.
However I think Tie-dus is better also because of the whole water/tide thing.
It's the same as the whole chocobo/chokeabo arguement..... Anyway people pronounce things different so maybe Tidus is just pronounced differently by different people!