stevenal - this is a case of current flow from a piece of metal, through a solution, then to another piece of metal. So a ground wire will help by giving the electrons an easier path. Applying a DC flow in the reverse direction will help even more (you could zinc electroplate/galvanize the...
Whatever the cause of the DC flow, you need to reverse it. Connect a DC source to the seawall and the boat and crank it up until your corrosion stops.
An alternative would be to try and give the stray current a better path - say a copper ground wire from the boat to the seawall.
