They won't. They won't ever have been on these ships. They won't know anything about them. And what is it going to save? The sacked employees are at least entitled to payment in lieu of notice, let alone redundancy payments, and much more likely compensation for unfair dismissal, because the jobs are clearly not redundant.
There is no way you can replace all your staff with agency workers and expect it to run, let alone something technical, with catastrophic consequences if it goes wrong, such as a ferry operation
Remember that maritime legislation is completely different to shore based legislation. I am pretty sure that this would be entirely legal (even though I don't like that fact) even if the vessels were UK flagged. The Maritime Labour Convention, which is effectively the main piece of legislation covering employment rights for seafarers really contains practically nothing about redundancy - all that is needed is 7 days notice, which no doubt will be paid out. All seafarers are effectively on fixed term agreements that last for the few weeks or months they are onboard.
It may well have been possible to bring some of the key agency staff onboard in advance, somewhat covertly. P&O have had various ships laid up over the last few years which would have made this easier. These aren't people off the street. By agency workers in this case, the only difference between them and the people they are replacing is who employs them. I am sure that they would have included people who previously worked on the ships too.
And of course P&O management would have expected the existing crew to lock in, which also gives additional time for the new crew to get familiar whilst this is all sorted out.
I would find it very surprising if they've been able to hire 800 qualified people without anyone noticing?
Surprisingly simple to do, especially at the moment with large numbers of ferries still laid up after Covid. A friend of mine was part of a similar team who took over the Isle of Inishmore from an Irish Ferries crew. It was all kept very secret until they were on the M4 going through Wales.
P&O's Twitter is now saying they are unable to run for "the next few days".
If this outcome took them by surprise that's some gross incompetence on the part of the management...
Of course it didn't take them by surprise, but they had to pretend!