Summary update
On the 11th March 2017, the French government issued a decree that will affect all seafarers living and domiciled in France.

From the 1st July 2017, any seafarer working on any vessel of any flag will be subject to compliance with the French Social Security System if they are either a French national or domiciled in France.

It will be the responsibility of the employer to deduct social security payments and pay them over to the French Authorities together with employer’s deductions, amounting to around 24% of gross salary.

Any crew member that pays into other countries recognised social security system such as the United Kingdom National Insurance scheme will be covered by this.
If the employer of the crew is not a domiciled French company or does not have a French agent, the French Authorities will require up to one year’s advance payment of the Social Security contributions to cover the employee’s and employers contributions.

Full article at : https://www.linkedin.com/pulse/changes-french-social-insurance-system-seafarers-steve-white?published=t