The New Zealand navy team trying to find the wreckage of the sunken Princess Ashika faces a "challenging environment" with depths of up to 800m, commanding officer Lieutenant Commander Andrew McMillan says.
A 15-strong team, comprising 12 drivers and a three-member remote search team, spent a second day today searching for the Tongan inter-island ferry, which sank on Wednesday with 149 people on board.
Two bodies and 54 survivors have been found, while 93 people remain unaccounted for.
Police say the final number of missing could be higher, and they were continuing to analyse information about unrecorded people on board the vessel, whose official manifest showed only 79 passengers and crew.
Mr McMillan said the team today focused on a 50-110m deep site where an oil slick and debris had been seen, and where the Tongan Defence Service's echo sounder had appeared to detect an object.
However, nothing was found.
"The topography of the seafloor is a very challenging environment here. It ranges from 35m down to 110m down to 800m," he said.
"You don't have to travel very far and the depth can change very quickly."
The New Zealand equipment had a limitation of 100m -"or, if we're very lucky, 115m".
"So with the topography, with the uncertainty of where the vessel has gone down, we have to face the realisation that we might not even be able to find it.
"But we'll certainly do our best."
The team would spend tonight aboard Tongan patrol boat and resume their search tomorrow.
Those members who remained in the capital Nuku'alofa tonight attended a multi-denominational remembrance service which attracted about 1000 people.
Tongan police commander Chris Kelley gave those there an update on the situation.
Earlier he said a complete manifest was held by a crew member on the ferry when it sailed but that had been lost in the sinking.
"What we are faced (with) is that people are telling us is they put people on the boat and they weren't on the manifest that was supplied here," he said.
Survivors have described how they saw the ferry hit by a 1m wave which swept the cargo to one side, causing the vessel to overturn.
"The ferry sank so quickly that no one was able to do anything, and I think the passengers inside just couldn't make it out in time because the ferry just overturned and sank so quickly, in a minute," survivor Viliami Latu Mohenoa said.
Mr Kelley said police and government support teams were visiting families throughout the kingdom "to try to confirm the exact number and identity of people on board".
Efforts were also under way to identify foreign nationals among the passengers.
"We know that there was one Japanese crew member, with two French and two German passengers missing - four people who were working in Tonga, and there may be more."
The two bodies recovered were of a British national who had been living in New Zealand and a Tongan woman.