The navy positioned the ship Lautaro near the abandoned Ushuaia in an attempt to prevent any environmental damage from leaking fuel.
But Bowermaster said it's still unknown how much fuel oil has spilled from the ship.
"A Chilean plane reports seeing no major leak, but it [has] also reported that a fuel leak has spread for half a mile around the ship," he wrote.
"Though containment efforts are being made, it is windy in the area again and the leak is spreading."
Ushuaia may not be able to free itself from the rocks, and has at least one hole, Bowermaster added.
"A sinking ship in this pristine, narrow channel would have long-lasting impact on both the local environment and the future of tourism along the [Antarctic] Peninsula."
"Accident Waiting to Happen"
In addition to the 2007 sinking of the M.S. Explorer, another ship—the Norwegian M.S. Fram—lost engine power during an electrical outage in December 2007 and struck a glacier, smashing a lifeboat but causing no injuries among its 300 passengers.
A boom in Antarctic tourism may be an "accident waiting to happen," Bowermaster told National Geographic News in 2007.
More than 30,000 tourists were estimated to have made the trek to Antarctica on some 50 different ships during the November 2007 to February 2008 cruise season, according to the International Association of Antarctic Tour Operators, a trade group.
"A big question for those who oversee and monitor tourism in Antarctica is [whether] there be limits on who can visit Antarctica, and on what kind of ship?" Bowermaster added.
The Associated Press contributed to this report.