Spokeswoman Joslyn Higginson says the activists are aboard the Algomarine in a bid to delay the delivery of nearly 30,000 tonnes of coal.
Higginson says the activists arrived at the vessel aboard an inflatable craft launched from the Greenpeace vessel "Arctic Sunrise," a former ice breaker.
She says after the boarding, the "Algomarine" captain radioed the Coast Guard to remove the activists, then veered away from the port to await the arrival of authorities.
Higginson says prior to boarding, Greenpeace members painted a slogan on the vessel's hull: "No coal, no nuclear clean energy."
The Nanticoke station is one four coal-fired electrical plants in Ontario that the Liberal government promised to shut down by 2007 a deadline that was later extended to 2014.