Are you sure you aren’t confusing genuine merchant seamen who are away from home for months with employees on a roll on roll off that pops back and forth across the Channel, Irish and North Seas?
I’m not saying you’re wrong but it seems a bit odd. Do lorry drivers not pay tax for the time they are driving around the continent?
Edit to add
If you're an employee working at sea outside of the UK check if you can claim tax relief on your earnings.
www.gov.uk
It seems to apply to people who have been outside the U.K. for at least 365 days.