But still irrelevant as maritime law applies on ships based on where they are registered not where they work to/from.
For employment matters, as P&O Ferries is a UK company, UK employment law applies. This is still the same as EU employment law. Maritime law is a complete irrelevance for employment law matters.
P&O Ferries appear to have broken several laws. They have not consulted prior to making people redundant. They have not given the Business Secretary 45 days notice that they intended to make more than 100 people redundant. There may well be other breaches.
Fire and rehire legislation would not have made any difference to this situation. They are not rehiring the workers who have been sacked. They are hiring agency workers and saying that sacked staff could join the agencies.