Yeah, we’ve been on 3 cruises with our kids in the past few years- all in the Caribbean. We are all vaccinated, and would not even consider it these days. Now, if the cruise line requires proof of vaccination, maybe. The main risk is the destinations not allowing the passengers to disembark because of someone popping on a test.
Some cruiselines are requiring pax be vaccinated, and some aren't. The CDC let those lines that are requiring vax to resume cruising without going through "test cruises" and a bunch of other hoops, while those that aren't sailing fully vaxxed had to go through all those protocols. Generally those that appeal more to families (such as Disney) are not requiring it, since the under-12s can't be vaccinated.
I can only speak fully to the one I've been following most closely, which is Celebrity Cruises. They started cruising again in June, and the requirements are a mishmosh depending upon where you're cruising. All of the following applies only to Celebrity cruises.
In general, Celebrity's requirement is that if you are 12 years old or more, you must have been fully vaxxed at least two weeks prior to sailing. They require you to take your original vaccination card to the port, where they check it and take pictures of it, etc. No exceptions if you're a teenager or an adult. Children under 12 can board if they present a negative test, which is administered at the pier. However, no more than 5% of the passengers can be unvaxxed, meaning no more than 5% of the passengers can be <12 years old.
In addition to this, they are requiring negative tests from everyone, whether or not vaxxed, if localities require it. For instance, they had some sailings out of St. Maarten, which requires tests from everyone whether or not vaxxed, so everyone has to get a test prior to boarding. In addition, they require a test if a locality requires it,
even if the person does not intend to get off in that port.
The exception to all of this is cruises from Florida, because the governor there will not allow businesses, even cruiselines, to require vaccinations. To get around this, Celebrity instituted a policy that they will not "require" you to be vaxxed, but if you are not able to provide original documentation of your vax before boarding, you will be subject to extra requirements that make sailing unvaxxed not very desirable. These include thrice-weekly testing at your expense while on board, special dining and entertainment sections for the unvaxxed, can only go on Celebrity-sponsored shore excursions (i.e., you can't go into a port on your own), and you must wear a mask at all times onboard except in your stateroom. In addition, they limit the unvaxxed to 5% on any ship (including children), so if you arrive at the pier and they've already reached that 5%, you're out of luck.
Regarding masking, for the most part the vaxxed are not required to wear masks onboard, but again subject to local requirements. The cruises out of Greece have been an exception where masking is more fully required.
Celebrity is using bracelets and "tracelets" to distinguish between vaxxed and unvaxxed so you can easily tell who should be masked, etc. And what happens if someone breaks the rules? Well, they get tossed off the ship, as an unvaxxed couple found out a few weeks ago when they went off the ship on a Celebrity-sponsored excursion but then took off on their own. The ship ended up leaving that port late while they took the people off the ship and worked with the local port authorities as to what to do with them.
"Tracelets" are used on some but not all cruises so far, but word is it will become the norm. These have a chip to record movements so that if someone turns up positive, contact tracing can be more easily done.
Has it been infallible? Nope. There have been, to my knowledge, three instances of a sailing so far in which someone has turned up positive. In the first, two people were found to be positive on the penultimate night of a cruise in the Caribbean, when everyone was getting tested in order to return to the US (Celebrity administers the tests on the ship). They were evacuated out on private jet back to the US, and those who had had contact with them were re-tested to ensure their negative results (these people were isolated in their rooms for about 24 hours). The second two positive are on ships currently sailing - one in Alaska and one in the Caribbean. In both instances, it was an individual person who again was airlifted back home (at Celebrity's expense, as this is their plan at least until November), and contact tracing found no other cases. In the case of the Alaska passenger, he/she had some mild cold symptoms so got tested voluntarily.
In each of these instances, the protocols worked and the rest of the passengers remained safe and COVID negative, and were able to continue their vacations. All in all, I'd actually feel much safer on a ship right now (or at least one from the lines that are requiring vax) than out at a restaurant or a grocery.