Here are some partners I thought up for if Super Mario Sunshine was a Paper Mario game.
FLUDD would act as the tattler and would be able to walk around on its own. In battle it would fight with water.
Mario would meet Piantina in Bianco Hills. She would have the same role as Koops with using her banana boomerang to hit far away switches and collect far away items.
Mario would meet Blooster in Ricco Harbor. He would be a racing blooper who can carry Mario across stretches of water.
Mario would meet Carlquack in Gelato Beach. He would be a laid back sunbather who can use his beak to pull away parts of the environment like how Flurrie can blow them away.
Mario would meet Bombit in Pinna Park. She would join Mario after Electro-Koopas create a blackout in the park, preventing her from playing games in an arcade. She would blow up cracked walls.
Mario would meet Booson in Sirena Beach. He would be a business Boo staying in Hotel Defino for vacation. He would turn Mario invisible like Bow.
Mario would meet Nokita in Noki Bay. She would ask Mario to help her find out the secret of the Noki Bay ruins. She would be able to detect hidden objects like Watt and Ms. Mowz.