I've had good luck with Starbrite Hull Cleaner for stubborn stains, including rust. If that doesn't work you could try Iron Out or a similar iron stain powder. Wear gloves with either of those products, and if you end up using one of the caustic powders, make sure you have plenty of ventilation - the fumes are really strong.
Joe
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USCG 50 Ton Master Inland/OUPV Endorsement+Towing
1998 278 Vista - Twin 4.3 GLs, Kohler 4KW Gen, AC/Heat.
1999 Yamaha Waverunner SUV
2003 Yamaha Waverunner SUV w/MR-1 HO conversion