Emergency oxygen and a first aid kit are on the boat on every trip and both are checked on a list before it leaves. Two of the staff hold current emergency oxygen and first aid certification and one is on every boat.
There is a written emergency plan naming the nearest recompression chamber, how long it takes to reach it, and who is called first. Ask to see it. Nobody ever does and it is on the wall.
The limits: a dive computer on every diver, a medical questionnaire before the first dive with a doctor's note for any yes, no dive within eighteen hours of a flight, and no repetitive dive plan that relies on being lucky. A guide can end a dive and does not have to explain it until the boat.