"General" Tidus outlines the rules for Island War: no getting wet, or else you're dead. As team leader of the Crabs, he picks Kairi and Wakka as his teammates, leaving Sora and Selphie for Riku's team, the Seagulls.
After persuading Selphie to betray Riku and join the Crabs, Tidus's team arrives at Riku's hiding place, the broken bridge, and tries to knock Riku into the water while staying dry themselves on stilts. Sora easily dispatches Selphie and Kairi, while Riku manages to knock Tidus and Wakka off balance, making the Crabs lose the game of Island War.
Later, Tidus and Wakka have a play-battle against Riku, two-on-one.
Tidus fights alongside Selphie and Wakka in a play battle against Riku, but all three are defeated easily.
Later, he loans a towel for Sora to give to Riku.
Tidus, looking for his towel, accidentally interrupts Riku and Sora just after they share the paopu. Mortified, he leaves quickly.