, I am 27 and a UK national working as a merchant navy deck

There is a principle in employment law where terms may become implied into an employment contract by ‘custom and practice’. This makes them contractually binding even if they are not written down anywhere. This area of law is rather complex and it is usually only down to the courts to establish with certainty if something had become an implied term. Nevertheless, it does not prevent employees from directly raising this argument with their employers.
The basic requirement for implying terms is the presumed intention of the parties, in other words - did the employer and employee intend for the terms in question to be treated as contractual. In general, a practice would need to have been clearly communicated and consistently applied for a substantial period of time before it can be considered an implied contractual term. Therefore, something that is uncertain, not communicated properly, not been applied consistently or has just been around for a few months is unlikely to qualify.
Case law has suggested that the following are important factors when considering whether a term has become implied in a contract:
• On how many occasions, and over how long a period, the terms in question have been applied - the more times they have been applied and the longer the period over which this has occurred, the stronger the argument they had become implied into the contract
• Whether the terms are always the same - large differences will make the argument they had become implied weaker
• The extent to which the terms are publicised generally - there must be widespread knowledge and understanding amongst the workforce that such terms were being applied
You may tell the employer that you believe the term or practice you are relying on, in this case the job you have been doing all this time, has been implied into the contract through 'custom and practice' and see what they say. They could of course deny that and refuse to discuss the matter and if that is the case then you can only realistically challenge this by taking your case to an employment tribunal, although you are free to raise a grievance with the employer first to try and deal with this internally.
I hope this has answered your query. Please take a second to leave a positive rating, or if you need me to clarify anything before you go - please get back to me and I will assist further as best as I can. Thank you