Vic, pressure wash as much off the hull as possible, however, after 3 months worth of build up, don't expect it all to come off. Then try a product called Sea-Safe Hull Cleaner found at CTC in the boating section for about $20 a bottle. It does a decent job on the hulls here on the west coast. If that doesn't clean it up, your next course of action will be Muriatic Acid...guarranteed to work but harsh stuff to work with! If you use the acid, you MUST use CHEMICAL rated rubber gloves and CHEMICAL rated respirator. These are not suggestions, they are a must!...don't cheap out, your lungs depend on it. If you use the proper equipment, it's safe to work with.
If you need any additional information, feel free to email me at cmacdetailing@yahoo.ca always happy to help out.
Good luck and I hope this helps.
cheers,
Mike