Nikon currently doesn't have any underwater housings in their product line. Here a is a patent* (2010-197939) for an underwater housing that can connect to an external tank of compressed air in order to prevent condensation inside the camera housing:
"The compressed air of the tank 60 supplied from one vent 27 scavenges the inside of the housing 1, and is exhausted outside from the vent 29 of another side. The air supplied from the tank 60 passes along the circumference of the lens (graphic display abbreviation) of a camera, and goes to the vent 29 for exhaust air through the crevice between a camera and housing 1 inner surface. Usually, the compressed air in the tank 60 is dehumidified in order to prevent generating in which tank 60 inner surface rusts. Since the inside of the housing 1 is scavenged with this air currently dehumidified, the humidity of housing 1 inside can be lowered. As a result, dew condensation of housing 1 inner surface can be prevented. Since the air in the tank 60 has the high pressure, the air of housing 1 inside interchanges for a short time, and the quick work of it is attained."
*Patents do not necessary mean that a commercial version of a given product will follow.