A rudimentary battery. Wiring is copper, coconuts contain weak acids and could be combined with the salt water to make an electrolyte solution. Then you state "other metal" besides the wiring. Dissimilar metals create an electric charge in the presence of an electrolytic solution.
They could theoretically create enough voltage and amps to power the marine radio on the ship to call for help.
Oh for sure. Even creating enough amperage to boost that out would be difficult, but they had all the coconuts, sea water, and time to experiment.
Steel shipping boats are essentially batteries themselves. To keep the sea water from rusting thy are outfitted with zinc bars around their hulls. The zinc works as an anode and the steel ship is the cathode. This creates an electric current in the water and instead of the steel rusting away, the zinc becomes a sacrificial anode and absorbs the oxygen while sparring the ferrous metal of the ship. Of course you probably know this though.
Benedict Arnold 