If this decoration is placed in a city, the ships and crops will never wither. Cargo planes should be safe. It is unknown if the effect applies green houses. According to Zynga it will not unwither the Super Pier.
It will only work on the city it's placed in. So if placed in your main city, it will not help your Downtown.
"Behold, the Eternal Fountain! Put it in your city and your crops and ships will never wither! Ever!"