GMP Tameside North released a statement asking anyone who may have seen Curtis Higgins since Sunday 31 July 2016, to come forward.
Curtis was last seen on The Stena Mersey - a Stena Line ferry - which sailed from Birkenhead for Belfast at 10pm on Sunday 31 July 2016.
The 29-year-old was last known to be wearing a black jacket, blue trousers and a black rucksack.
He is white and is described as having short dark hair, about 6ft 4ins and of a slim build.
Officers believe he may have spoken to two men once on board who may have information about his whereabouts.
One man was wearing a hooded top and jeans whilst the other was wearing a dark hooded top and dark shorts.
Detective Sergeant Thomas Howard, from GMP’s Tameside borough, said: “We are extremely concerned about Curtis and want to make sure he is safe and well.
“We are keen to speak to the two men described above who may have spoken with him or noticed him prior to him going missing.
“These men are not believed to have any involvement in Curtis’ disappearance but may have key information which could help us find him.
“I would urge them, as well as Curtis, to please get in touch.
“If anyone else thinks they have seen Curtis since Sunday 31 July, I would also ask you to please contact us.”
Information can be passed on to officers by calling 101 and quoting incident number 566 of 2 August 2016.