If you are employed as a mariner on a British ship and you are domiciled or resident in the UK you will have to pay Class 1 contributions.
There are special rules for mariners resident in another EEA State but working on a British flagged ship
Your employer, or the person paying your wages, will also have to pay Class 1 contributions if they live in or have a place of business in the UK.
If your employer or the person paying your wages does not live in, or have a place of business in the UK, they will not have to pay Class 1 contributions.
If you work wholly or mainly within Category A, B, C or D waters and your employer supplies you to work for a business in the UK, then you may have a 'host' employer who will be required to pay Class 1 contributions.
If there is no employer, payer of wages, 'host' employer or other secondary contributor in the UK to deduct your Class 1 contribution from pay via a PAYE scheme, then you will need to pay your contributions directly to HM Revenue & Customs. Your contributions will still count for benefit.
From 1 May 2010, an employer in another Member State of the European Union
is treated as having a place of business in the UK for National Insurance
contribution purposes and National Insurance contribution rules apply to that
employer, where their mariner has to pay Class 1 National Insurance contributions.