The Offshore Platform is a city improvement present in the game Civilization II. Much like the harbor improvement, the offshore platform was added to make up for an imbalance in the original game.
The offshore platform gives an additional production shield to each square of water in the radius of the city it is built in. Since most ocean squares don't produce any resources, this turns the ocean into a large source of resource. If a city is surrounded by water, building an offshore platform can transform it from being unable to produce for itself into a powerhouse spewing out military units.
The Offshore platform can't be built until relatively late in the game, when your civilization has the advance of miniaturization. It is also somewhat expensive to build in resources.
The largest issue in building offshore platforms is that most cities that can build them don't need them, since they already have a great deal of production power, and that cities that need them can't build them. Of course, by the time you gain the techonology of miniturization, you have enough money to buy the platforms if needed.
Offshore platforms can be very important in the endgame of Civilization, especially if you need a production boost against an opponent.