Sadly I don't think you can eradicate the threat of Covid (or even Noro) completely. DH, adult DD and myself sailed with Cunard to the Canary Islands over Christmas and New Year but it was only towards the end of the cruise did we become aware that Covid was present. In fact, the couple in the cabin next to us had it and were isolating in their balcony cabin. A table companion also mentioned that several cabins on her deck were also isolating.
But here's an interesting fact: The 3 of us shared a cabin and were together virtually 24/7. DH is classed as clinically vulnerable and has had 5 jabs, my daughter and myself have had 4. On disembarking we shared (obviously) the car journey home of about 5 hours, arriving back late afternoon. The following morning daughter woke up feeling 'off', took a test and yes, it was positive and remained positive for the next 3 days. We immediately took a test and guess what, both of us were negative and continued to show negative for 3 days. We then stopped for 24 hours before testing again (out of curiosity) and were still negative.
So why did she get it and despite being together all the time, why didn't we?
There is no answer to the above question but for us we simply came home and have booked 2 further cruises for later in the year!!!
One final point for the OP: some people wore masks all the time but the majority, including us, did not. The only time Cunard asked us to mask up was when we used the shuttle buses but even then some people did not bother.