Sorry, but it won't work. I'm not sure which would happen first, but you would either ignite your fibreglass or toast your engine. The fibreglass hull will not adequately conduct the heat out of the system no matter how cold the water is.
What you COULD do, and many engines particularly those on work-boats have been configured this way, is run the closed coolant loop OUTSIDE the hull and create what is called a "keel-cooler". They work extremely well, but most sailors find the additional drag unacceptable (as opposed to motorboaters that are always under mechanical propulsion.)
I commend you for thinking outside the box. However, as imperfect as the heat exchanger may be, it's not so bad either.