P&O has dismissed claims staff are refusing to go back on board The European Causeway, after it lost power off the coast of Larne on Tuesday afternoon.
It comes as ITV News revealed it had seen documents suggesting staff could be earning as little as £3.94 an hour.
P&O, however, said that was misleading and no staff were paid less than £5.58 an hour.
Inspectors from The Maritime and Coastguard Agency spent Wednesday on board the ship, investigating what caused the mechanical failure.
Micky Smyth from trade union Nautilis said he wasn't surprised by the power loss incident.
"When you replace maritime professionals with experience you expect these situations to occur," he said.
"We have received information from seafarers on board the vessel that they have serious concerns about health safety and wellbeing onboard." A spokesperson for P&O Ferries said: "The overwhelming majority of agency seafarers onboard all our vessels are working well, as normal. "Claims concerning seafarer discontent onboard the European Causeway are categorically false and entirely baseless. "Following a temporary mechanical issue on the European Causeway yesterday, we expect the vessel to begin sailing again in the next 24 hours, and look forward to welcoming passengers as normal."