David Smith claimed the comments were "banter" and a "joke" and thought his remarks were private. But he was dismissed when bosses at Scottish Canals took the view that he had been consuming alcohol.
The 34-year-old, who told an employment tribunal he believed his Facebook page was private, also posted abusive messages about his supervisors, calling them w*****s, p****s, and c***s.
Mr Smith, of Lambhill, Glasgow, claims he was being bullied by his supervisors and was about to raise a grievance against them when the comments, which had been posted two years earlier, were e-mailed to Human Resources.
He believes they were brought up to prevent him making his grievance, which also related to health and safety issues.
The comments made by Mr Smith stated: "On standby tonight so only going to get half p***ed lol (laugh out loud)."
When asked by another internet user what he was on standby for, he replied "Flood alert", before commenting: "Drinking vodka and apple juice, first time I've tried it, not too shabby."
Another post stated: "Why are gaffers such p****s? Is there a book teaching them to be total w*****s? Need a new job."
Mr Smith told the tribunal he believed his Facebook page was set to private and would never have made the comments if he knew they would be in the public domain.
Asked about the drinking on duty comment, he said: "The comments were made two years ago. They were banter and a joke. I would never drink alcohol while on standby and never have done."
However, Phil Martin, head of Business Improvement at Scottish Canals, who conducted the disciplinary investigation, said he believed Mr Smith had been drinking.
Mr Martin said: "He bragged about being drunk on standby. It was very specific, it wasn't just I've been having a drink, it was this is the particular drink I'm drinking. He was putting the people of this area at risk. We could be sitting here today with somebody dead because of him - that's how serious it was."
HR manager Fiona MacMillan also told the tribunal that the other derogatory comments were highly offensive and brought Scottish Canals into disrepute.
Contracts of employment often contain express provisions allowing employers to terminate an employee should their conduct outside work bring the employer into disrepute.
Mr Smith, an operator at the body's Forth and Clyde West team, argued that he had not identified himself as an employee of the organisation on his page.
But Ms MacMillan said a comment he had posted on another employee's page referred to BW, the initials of British Waterways, the previous name of Scottish Canals.
Lawyer Lorna Davis, representing Scottish Canals, put it to Mr Smith that the company had formed the view he had been drinking due to the level of detail he went into.
Mr Smith replied: "You could write anything on Facebook, it doesn't mean that you're telling the truth."
Ms Davis added that the drinking post attracted 82 comments from other Facebook users, asking: "When people comment you have no control over their privacy settings so this could be out there in the public domain, is that correct?"
Mr Smith replied: "Yes."
The solicitor also asked if he knew how to check his privacy settings on the social media site and he replied that his wife checks that for him.
Asked about how he felt when he realised his page was in fact public, Mr Smith said: "I was absolutely horrified when I found out my page was public, not only because of comments I had made - there were pictures of my children in the bath there too.
"Images which I thought were private. It made me feel sick."
A judgment will be issues in the tribunal in the coming weeks.