Due to electrolysis, boat hulls and motors can be damaged by the way of galvanic corrosion.
This occurs when a vessel is situated in an electrolyte (fresh or salt water) and has two dissimilar metals. The metals become electrically connected via the electrolyte and a natural chemical reaction takes place.
This forms a circuit where the least noble metal (the anode) breaks down, leaving the more noble metal (the cathode) intact.