Have you considered maybe using a pneumatic air chisel on the water pump? The vibration from it might soften the bond between the 2. Which brings on another question being it the block still usable with that much corrosion on it? I don't envy you having to get all that stuff apart. A boat that was prior neglected in freshwater is bad enough, but when you add saltwater to the equation, the carnage is 5x worse. As expensive as these boats are (new), I just don't know how these owners can just let them rot away like that. Glad to see you are putting new life into one.