I was on a dive off of Boynton when they partnered me up with a female. We both entered the water and held onto the the line. She said she couldn't breath through her regulator so she asked me to check her tank valve and I did, and it was closed, so I opened it for her. She obviously had enough buoyancy to stay afloat, but more importantly, her dive buddy (me) was there to help her out. I opened it back up and we descended.
Things on a dive boat can get really hectic for the last 15 or 20 minutes before people are going to start stepping off the boat to dive. Everyone is trying to get their gear ready and on and there is hardly any space to move around on a full boat. The boat crew are trying keep things moving because nobody wants to lose dive time because someone in front of you trying to exit the boat is not ready or are having problems with something. Nobody wants to be that guy that delays everyone. In all the chaos, people can lose focus and forget the basics. I can totally see how it can happen to someone to enter the water without their valve on, but if you habitualize your prep routine, it helps. I'm OC when it comes to some things like locking the door to my truck multiple times when I walk away from it, or checking a couple times to make sure I have my wallet or checking multiple times to make sure my stove is off after I'm done cooking. I have the same OC behavior when I'm on a dive boat with things like my air valve and taking a few breaths through my regulator or checking my pressure gauges. Find a system that works for you and stick to it.
Forgot to say, all my dive tanks , except for my pony bottle, have a vindicator valve that gives you a quick visual if it's open or closed. Of course this doesn't help me when I'm on a dive trip and have to use a rental tank, so it's something I have to be mindful of in those scenarios when I'm out of my routine.