An aerial inspection yesterday revealed there was a stable 1.7km-long oil slick in the sound.
The Fiordland Cruises tourist boat sank with an estimated 700 litres of fuel on board.
Doc rangers said Fiordland crested penguins and shags were the creatures most vulnerable to oil in the area.
Environment Southland spokeswoman Michele Poole said trained seabird rescuers were on standby.
However, there was no indication yesterday they would be needed, as computer modelling predicted the slick would naturally dissipate by tomorrow.
The oil slick was 5.5km from the nearest marine reserve and about 10km from the nearest known breeding area of the Fiordland crested penguin, Ms Poole said.
"The oil slick is confined to a 1km area centred on the sunken boat. Given that 16 hours have passed since the vessel sank, we are increasingly confident the slick will not reach the most sensitive areas, in terms of wildlife."
The ship's occupants, two crew and eight tourists from Britain, Germany, Switzerland and Ireland, were on an overnight fishing and birdwatching cruise in Doubtful Sound.
The unnamed skipper of the boat had 30 years' experience and had worked for Te Anau tourist operator Fiordland Cruises in Doubtful Sound for the past three years, Fiordland Cruises co-owner Clint Tauri told the Otago Daily Times.
"I respect [his] judgement ... He has expressed his view it was an incident and we are talking to the authorities."
The 36-year-old Waverley struck an obstacle at the Shelter Islands, on the western side of Bauza Island at 6.30pm, Mr Tauri said.
"They immediately headed back up the fiord to Grono Bay, which was in the [Te Awaatu] marine reserve. From there, they transferred their eight passengers to two vessels alongside."
Deep Cove Charters boat Seasinn took on five Waverley passengers and Real Journeys boat The Navigator took on three.
The evacuation took about 15 minutes and the respective boats continued on their own overnight cruises, with meals and refreshments offered to the new arrivals.
Deep Cove Charters co-owner Diane Lemin said her husband, skipper Chris Lemin, and their son Travis (17) were on board Seasinn with seven passengers.
"Chris dived on the Waverley all afternoon from when it happened to patch it up, then tried to bring it into Deep Cove to put it on a slip for repairs, but it sunk off Matai Island," Mrs Lemin said.
Mr Tauri said Mr Lemin, in wetsuit and scuba gear, identified a "baseball-sized bruise" on the Waverley hull.
Despite the temporary patch and tow, about 10.30pm, Waverley was taking on more water than the pumps could manage, Mr Tauri said.
Mr Tauri and co-owner John Harris flew to the boat in two helicopters from Te Anau with extra pump gear and arrived about 7.30pm intending to airlift passengers back to Te Anau.
However, they were told the passengers were content with their alternative tours.
Mr Tauri and Mr Harris decided to leave the 15.5m-long boat to its fate and watched it slowly sink in about 300m of water at 12.05am yesterday.
Waverley passengers from both boats were taken by a Fiordland Cruises bus back to Te Anau.
Mr Tauri said Waverley was the first overnight cruise boat to operate on Doubtful Sound.
"It's a shame a bit of Fiordland history is gone."
Te Anau police said the incident was not reported to officers and they would not investigate.
Maritime NZ was making preliminary inquiries but had not decided whether to investigate.
