Police boarded a Stena Line ferry from Liverpool to Belfast on Friday and detained several passengers for questioning following claims of a blood-soaked brawl on board the vessel.
Shocked passenger Kris McCallen described what happened on board the Stena Line vessel, saying his children were "really frightened" by the incident which happened on board.
"Gotta love it when you're bringing your family back home and a huge fight happens, blood everywhere, my children were really frightened," he wrote.
"Was a lot of men fighting and falling all over the floor, blood gushing out of a guys head, someone screaming 'that's my brother' then charging!
"Staff really hadn't it under control at all. Is there no drink limits on a boat? Lots of kids here having to witness that," Mr McCallen, a musician and sound engineer, said in a social media exchange with Stena Line UKIE officials.
"My kids aren't happy at all, I'm not happy at all.
"I thought that Stena provide safe journeys for people," Mr McCallen said.
Last night, Stena Line UKIE confirmed that a police investigation into the incident was under way, and that some passengers had been detained by police for questioning.
In a statement issued last night, the company said: "Following a serious incident today (21st December) on the Stena Mersey 10.30 am sailing from Liverpool to Belfast, Stena Line informed the PSNI prior to the vessel docking in Belfast of the situation and currently a number of people have been detained for questioning by PSNI officers.
"The incident was quickly brought under control by Stena Line staff. Stena Line is currently reviewing CCTV footage of the incident and is helping the police with their enquires.
"Stena Line would like to apologise to other customers who had the misfortune to witness the incident and would like to reiterate its zero tolerance policy when it comes to unacceptable behaviour towards customers or employees. An internal investigation has been initiated.”