I have used starbrite hull cleaner for years with good success. Key is to spray or wipe it onto a dry hull, and within about 1 min. you hull should be bright white again. Keep it off of anything galvanized though, it will eat through that with aggression. It is pretty toxic stuff, so wear eye protection and gloves.
This year I am trying something new I researched on the interweb, ordered powdered oxalic acid and plan to dilute that in some water and see how it works. Might be a less toxic and inexpensive option compared to the hull cleaners. If I have success I'll let yall know.