Hello mpn 1cxe,
Xiankai has a very good idea, paint, or epoxy a protective covering to prevent further corrosion. The white paste-like substance is most-likely aluminium chloride. The reaction is electrolysis at the molecular level. If electrolysis did not occur, neither would the corrosion reaction. The Al2O3 protective layer does slow down the corrosion reaction, but does not stop it completely. Before painting or applying epoxy, ensure that all materials are completely dry and salt-free. This will minimize corrosion from increasing its rate one the cover is applied.
Any mechanical motion will eventually remove the protective Al2O3 layer and will expose 'fresh' aluminium metal, which can form aluminium chloride. The Al2O3 layer can be as thin as 1 molecule. With a layer this thin, even metal stress from the rocking motion of the boat will expose Aluminium metal.
Best of luck with your boat,
Eugene Dakin Ph.D., P.Chem.