There are special rules for employees who are classified as seafarers and providing that certain conditions are met, it may be possible to claim the seafarers' earnings deduction.
In order to qualify, you must be working under a contract of employment as the rules do not apply to self-employed individuals. You must be working on a ship - oil rigs and offshore installations do not qualify, but cruise liners, tankers and passenger vessels do qualify as ships for this relief. The other main condition is that you must be working wholly or partly abroad - this means outside UK territorial waters.
You must have a qualifying period of at least 365 days in order to be eligible to claim the relief and you must spend more than fifty percent of that time outside the UK for that period. Days of departure from and return to the UK must be recorded and any documentation to support those dates should be retained. You must record all ports of call for each voyage, as well as details of each ship you work on.
Claims must be made on a self assessment tax return each year and there are special sections that must be completed in order to claim the relief. Providing that a qualifying claim is made, the earnings during that qualifying period are not subject to tax in the UK.
This is a complex area of legislation and you should seek advice about your own personal circumstances to ascertain whether or not you will be eligible for the relief.