does anybody know how to get rid of the white stain on the sides of my fiberglass boat where it was sitting in the water for a long period of time?

My friend has a Baja and it had some red stains on it, not sure from what and he had it cleaned and those stains came right out.

There are lots of products and techniques for removing stains of different types.  "White" stain has me a little confused, are you sure it isn't oxidation (like a cloudy look)?

Put up a pic and we can give you the resolution or at least a good place to start.

If it's been in salt water, it might be the salt reacting with the gel coat. Try some "GoJo" or "Goop" hand cleaner (no grit) and a 3M (green) Scotchbright pad (you can get them in the grocery store). We cleaned a friend of mine's boat with this and it came out like a new nickle.

Ronnie

Zing makes a product, i know that my aluminum boat was brown from fishing in muddy waters, and it made it look like brand new aluminum.  All you do is brush it on and hose it off, but it is very toxic.  That was on my aluminum boat, Zing also makes a fiberglass formula.

I wouldn't reccomend treating it till you know what it is.