If salt water is in places it's intended it slowly dissolves the surfaces. The water jackets on engines intended for raw water cooling, have enough meat in them to provide many years of service before they rust through. If the water is in places it isn't intended to be, like the crankcase and cylinders, you need to get the water out fast and "pickle" the engine until service can be done. If you try to start an engine with water in the cylinders, she'll hydro lock and you'll likely have to replace the engine, if not most of the internal parts will need replacement.
There are a number of places you can get GRP in sheet form, most ground to a specific thickness. This is a rather expensive way to build things in 'glass. The nature of typical 'glass work allows the user to create objects of near any shape, you start with goo and reinforcement and end up with a hard part.
There are many books on the subject of 'glass work available. Log on to
www.westsystem.com and have a look at an industry leader in reinforced plastics. They have free booklets covering their products and techniques to use in your project. They are just an epoxy manufacture, but you'll learn much.