Do you guys have cillit bang?
http://cillitbang.co.uk/index.shtml (it was easier than a pic)
I use the lime one neat with a cloth and just rub the boat down with it after I've washed and dried it, and then rinse it off. Not sure what it could do to the gelcoat though so I only use it occasionally and sparingly, but it leaves the boat looking new.
Can't help you with the tire marks though.