How many times have any of us been on a night dive and discover a diver with only 10 dives under his/her belt going on their first night dive? They've barely figured anything out and now they are adding darkness to the mix? That's so dangerous to me.
I see this sport being taken way too lightly by way too many involved in it.
The reason why I take this "sport" so lightly is because it's pretty safe, fewer deaths than bowling IIRC, and most people don't take bowling too seriously either. Yes, occasionally people drift off, occasionally people get bent/embolized, and I suppose a shark eats someone every now and then. But the fact remains that the vast majority of recreational dives are conducted incident-free, where the worse that happens to someone is a hydroid sting or a bloody nose from a clogged sinus.
My first post-certification dive was a night dive, in mediocre visibility and 55 degrees, off San Clemente Island, a dot about 50 miles off Southern California, where the nearest land to the west is Hawaii. I got separated from the group, had an uncontrolled ascent, and suffered through the nausea of alternobaric vertigo as I kicked my way back to the boat alone. But I saw my first ever moray eel, cool!
Since then, I've faced a lot of other dangerous situations underwater, stuff that the average Caribbean cruiseship diver will never experience. I somehow managed to survive them all without harm. If diving is so safe that it hasn't killed me yet, surely it's not worth it for the average Caribbean cruiseship to be overly concerned.
But I'll agree that some "certified" divers really shouldn't be in the water without close supervision. Certification standards are probably too lax. Unfortunately, statistics don't bear out much causal relationship between low standards and diver injuries/death, so the standards don't change (or get even lower). Until divers start dropping off like flies, you'll see the same barely certified divers doing the same [potentially dangerous] Cozumel dives and probably having a great time.
When I got a bit unnerved during the one down current I experienced in Cozumel, my friend had a sister who had just finished her cert dives. He got separated from her when the crazy currents hit and hence became even more unnerved than me, thinking he had lost her forever. To his surprise, when he later found her back on the boat, she had loved the dive and didn't even notice the downcurrent. Go figure.
Whether it's the duty or just the moral obligation of a DM to keep his group safe, I'm not certain. It depends on each party's expectations (is the diver expecting a critter spotter or a hand-holder), the level of expertise of both (for instance, I've had DMs with far less experience and training than me guide me on dives), written documents, and agency guidelines. However, it is in the financial interest of every dive professional to keep divers safe for the selfish reason that, if divers get injured or killed, standards might get tougher, there might be scrutiny from government, and undoubtedly public interest will wane and pro divers will lose their jobs. Any DM in Cozumel who loses a diver should be shunned, booted off the island as an incentive for the rest to keep their divers safe and their pockets full.