"The conditions... are very difficult," said Roger Tupper, director of Hong Kong's Marine Department. "It is absolutely black."
"The problem with this wreck is, when it sank it turned upside down... the challenge has always been: is there any way to get in at any level?" he said.
The Ukrainian-owned tugboat Neftegaz 67 sank late Saturday when it collided with Chinese cargo ship Yao Hai in waters northwest of Hong Kong's outlying Lantau island.
The 80-meter-long vessel sank quickly but the Chinese ship suffered only bow damage and stayed afloat, officials said.
Only seven of the NefteGaz's 25-strong crew, including the captain, were rescued and were being treated in local hospitals.
The Neftegaz 67 was detained in Hong Kong in September 2003 for safety problems, according to documents from Hong Kong's Marine Department, the Associated Press reported.
The documents said that the ship did not provide a means of escape or escape breathing apparatus, and that the vessel's crew were not familiar with safety procedures.
It was not immediately clear if those problems were addressed during the vessel's detention.
Anatoly Prisiazhnyuk, the president of energy company Chornomornaftegaz, which owns the tugboat, was reported as saying that the Chinese vessel had failed to give way to the tugboat before the collision.
Speaking from Ukraine he said the tugboat's captain told him over the phone that marine authorities had authorised him to pass.
"The Chinese cargo ship should have let it pass, but it did not do that," Prisiazhnyuk was quoted as saying.
The vessel had been travelling from the southern Chinese city Shenzhen to an oil field south of Hong Kong.
Hong Kong officials have said that rescuers will continue their efforts until the Ukrainian vessel is retrieved and the bodies are located.