HI 

Rust stains on polyethylene are purely cosmetic as you pointed out and usually come out with mild cleaners. I would suggest starting with vinegar or a gentle scrub (like a VIM), and if needed, move up to something like Barkeeper’s Friend or a diluted rust remover. Always use a soft cloth or sponge and rinse thoroughly afterward. Avoid harsh solvents or abrasives that can scratch the hull.

Hope the helps,