I went on the Roatan Aggressor liveaboard around the end of Oct, not too much after the country opened up. They did require a negative covid test with a sensitivity of 98% or higher 72hrs prior to landing in the country. Long story short, I eventually got a rapid test that was actually the PCR kind and in compliance with the requirement. Aggressor had an extensive covid-19 policy, and it was very strict, including no pre-boarding stays or excursions, pickup of passengers by their own hired van from the airport, mask indoors and social distance all the time, etc. During the onboarding orientation, they said they take it very seriously, and all it takes is one outbreak and they will be done with. But in reality, some of them are followed, while others are not (no van came to pick us up, other passengers seem to have had braided hairs, indicating a visit to a braider, no masks requirements for the passengers, and on some other reviews, they went on land tours on the day before disembarkation.) I presume they get more and more lax based on what they think is more of a risk, since on some earlier reviews, there were mentions of wearing a mask while masking up. We had some anti-maskers that did not wear their mask on the van trip back to the airport until demanded by the driver.
As for if you think you should isolate yourself if you become sick, it is probably too late for everyone else. And they won't just put you in a room, but the whole trip will probably be aborted.
Even with the requirement of a negative covid test, it is not foolproof. We felt concerned when we were all jam packed into the little airplane flying from the mainland to Roatan, and many people in there were locals that did not require the covid test, nor were most of the passengers on our NY to miami leg of the trip. You can also look at the first ever post shutdown cruise (they called it chartered yacht) sailing in the Caribbeans. Their protocol was much more extensive than any liveaboard, including pre-flight testing, pre-boarding testing, pre-disembark testing, daily temp check, complete bubble from airport landing to takeoff, passengers count so low that they can easily social distance. The only non-requirement was masks. They have also been sailing successfully all summer in Europe. All it took was some selfish group from the US that partied like there was no tomorrow on their stayover in Miami before boarding, and they whole group, along with another party of 2 and some crew members got sick/caught it, thus cutting the whole trip short, and literally shutting down that industry in the Caribbeans for the next year or so. I don't know if it was the 'no-masks' requirement that attracted the non-compliancers to that trip, thus along with people not taking precaution. But on my liveaboard, we had passengers and crew who believed that too.