Seafarers' tax and the 100% Seafarers' Earnings Deduction
Individuals regarded as resident in the UK (and non-UK resident but EU/EEA
resident individuals since 6th April 2011) employed in any capacity on a vessel, (whether it be as a captain or a deckhand on a superyacht or a stewardess or an entertainer on a cruise ship, as security or as an engineer on a boat in the anti-piracy or oil industries, or a chef in the merchant navy) are entitled to claim the seafarers' earnings deduction to exempt their earnings only from UK tax, where they qualify. However, other worldwide income and gains will remain liable to UK tax.

For a seafarer to qualify for the deduction, a qualifying period of at least 365 consecutive days needs to be set up, which can include non-work days. This period can straddle more than one tax year.
Duties are regarded as being performed outside the UK if they are carried out on a vessel where the voyage or part voyage begins or ends outside the UK. The UK sector of the North Sea is treated as part of the UK. There must also be at least one foreign port visit in each employment in each tax year.
Visits to the UK must not exceed 183 consecutive days nor one half of the qualifying period by counting midnights. For the latter, the period concerned runs from the first day abroad to the last day of the period spent abroad, after the previous visit spent in the UK: