But divers who knocked on the hull of the tug, the Naftogaz 67, did not get a response, they said.
Rescuers are working to stabilise and then move the vessel to allow divers better access.
Seven people were rescued after the vessel sank, but the remaining 18 are thought to have been trapped in the ship's engine room and cabins.
Strong currents
The Naftogaz 67 collided with the Chinese-registered cargo ship Yao Lai late on Saturday near the island of Lantau.
It sank almost immediately and is currently on the sea bed at a depth of 37m (120 ft).
Divers took six hours to locate the vessel, which may have been moved by strong currents.
Hong Kong marine department director Roger Tupper said efforts were now focused on moving the ship to shallower water so that divers would have a better chance of gaining access.
But a spokesman for the Guangzhou Salvage Bureau described the sailors' chances of survival as "very slim".
The cause of the collision remains unclear.
