I've always thought that it MUST be "Tide-us". So like, you can hear the word "tide" in there. I've always associated Tidus with water, since he is one of the three "underwater" characters, he's a professional aquatic sport player, and his Brotherhood weapon, when powered up, deals water-elemental damage.
Similarly, I associate nearly all Final Fantasy characters with elements. It's a strange habit, come to think of it.