The crew is always on duty. What if the boat were to have an emergency. I certainly wouldn't want some pot head trying to figure out what to do. Under no circumstances should a crew member be drunk or under the influence of any substance at any time. Notify the captain, owners and potential customer via Scuba Board.
While every country has its own maritime laws, the USCG is very clear about crew members being under the influence of drugs or alcohol on passenger vessels. Crew members are subject to random drug testing (and all members of a crew are tested if there is an incident onboard, even if it is relatively minor). Crew members are not permitted to drink for 8 hours before boarding the vessel, and blowing a .04 on an alcohol test is grounds for disciplinary action (as opposed to a .08 for driving under the influence in automobiles). While these laws are specific to US flagged vessels, it is easy to understand the rationale for enforcing these laws (as well as companies enforcing their specific rules for employment).
Let me give a good example of why these rules are necessary, at least with US flagged vessels are concerned. Many years ago one of our crew members decided to go by skiff onto an island during his "off" time. He went to the bar and had one shot of tequilla. Some of the guests of the vessel saw him do the shot. Later that evening, when this crew member was running the tender back and forth between the vessel and the island there was a collision with another tender. One guest was SERIOUSLY injured. The crew member swore he had only had one shot of tequilla (and i had no reason to doubt his claim), but because guests had seen him do one shot, the immediate assumption was that he was drunk when the collision occured. The resulting injuries were horrific, and the company ending up paying a substantial amount of money. Bottom line, there is (and should be) zero tolerance for drug and alcohol use on US flagged vessels.
I realize that the original poster was not talking about a US flagged vessel, but ask yourselves, do you really want someone who MIGHT be high on something being responsible for your health and safety? I'm not just talking about major onboard emergencies, but little things like picking up and towing a diver in a skiff, or filling your tanks, or just "watching your bubbles" to see if anyone surfaces in distress.