I also have questions about this situation. We are departing from the UK, on the Jade in September. I am hoping that things will have changed by then, but can't guarantee. I was told when I booked, that we needed to test 72 hours before boarding, and that would be the only test required. I only found out later that for Greece, a further antigen test is done, by NCL, actually at the port.
That's pretty clear, but what is not clear, is what do you have to do with that 72 hour test? Upload it somehow, or just take it to the port. I then assume (Greece only), they are concerned you have picked covid up in the intervening 3 days, and test again. In which case, as you say, why not just turn up and get tested at the port. Or will they not let you onto the port without showing the negative test? I note that for cruises departing US ports, the test has to be done no more than 48 hours before.
These rules are so random, and so confusing, it wouldn't surprise me if there are many people get it wrong. And of course, if you turn up at the port, having previously tested negative, then subsequently test positive, what on earth do they do with you. Refer you to the authorities who ship you off to self isolate, leave you to sort yourself out, or what? Ive asked this question on live chat, but they refer you to the cancellation/refund policy, which isn't quite what I am asking. My cruise consultant, who is otherwise very helpful, didn't appear to know that this extra test at the port was required. Of course I get that with so many different rules for different countries, it's hard for them to have all this information at their fingertips.
It would be wonderful, for us who are cruising after you, if you'd post back on here, exactly how this works.