Anything with oxalic acid will work. I am not sure if the active ingredient in On-Off is oxalic acid but if folks say that works then try it.
Magic Erasers are more abrasive than most think and can easily wear through gelcoat. I only use them as a last resort.
Now for the 'bad' news. If you have rust running down from the swim platform where it connects to the hull ( screws, bolts, etc), that means that water is entering the screw holes. The best approach is to remove a bolt and see if there is a lot of moisture. Hopefully, the problem is local to the bolt holes only. The bolts or screws should be removed and reinstalled with plenty of sealant such as 4200. The goal is to make sure that moisture does not enter around the bolt. Note that even stainless hardware will rust if wet and NOT exposed to air.