As a dive guide,I have been in charge of many,many 'divers' from curise ships. It is a very difficult,timed,and regimented dive operation. Guiding up to 60 divers a day on cattle boats is the only option when it comes to volume diving. My suggestion is if you are not on the big white boat,do not get on the one that's servicing those 'divers'. It came be quite a circus,confusing and intimidating. As to diver per DM ratio,there is no rule or standard I know of. Safe diving is up to the certified diver.
"living life without a hard bottom"
KT