You can make a functional battery cell by placing zinc and copper electrodes in salt water, but it's not very efficient. These are direct generating cells that use electrolosis to create a low voltage current, which means they become even less efficient over time as the zinc electrode slowly vanishes. The system probably would have some utility in a lighted buoy, since it could use the ocean itself as the electrolyte.

Any dissimular metals in salt water will create an electric current. This is why you place heavy zinc anodes on the propeller shafts. Otherwise the less "noble" metal, which is generally going to be the hull, will slowly erode. The zinc, being less noble than steel, becomes the victim instead of the hull. This is also why museum boats normally have their screws removed if they're in the water.

Submarines do get the water to top up their batteries from sea water, but only after running it through a distiller. Additonal acid is never added to a battery, only pure distilled water. Any impurities in the water reduce battery life and efficiency -- which is why you should never use tap water in a car battery -- and salt water has the unfortunate property of generating poisonous gas when it mixes with the electrolyte.

In American fleet boats, you could generally tell the junior electrician's mate by the acid burns on his dungarees. Low man on the totem pole usually had the job of checking the cells and topping them off.

J.T. McDaniel