We would need to travel mid June from UK to France by air and at the moment we understand that from June 8th France would require us to go into quarantine for 14 days. This is a work trip so that would not be an option for us and we would have to cancel. If we had a seamans pass/book would we then still have to quarantine or not?
There are several exemptions, of which two might apply:
crew members and people operating merchant and fishing vessels;
people allowed to enter France for economic reasons if they are staying for less than five days.
Note that, as stated, the exemptions are based on the purpose for the trip. Merchant vessels crew members typically have a seaman's book but you wouldn't be allowed to go on an unrelated trip merely because you hold the document (outside of the Covid-19 situation, this is how it works for visa exemptions too).