Just because Tidus only just started using a weapon doesn't mean he's weak. In fact, it's probably because he's a good blitzball player that he's stronger than Kimahri. Remember the FMV with the blitzball game, imagine what kind of strength he has that he can swim to the top of the blitz sphere and hurl himself that high out of it. Just because he recently started fighting doesn't mean he's weak, and it's not like he's doing anything fancy with his sword.