I love this idea on principle, but I wonder about situations where touching is expected? At least equipment touching (e.g., shared-air exercises and tired-diver tows). Less applicable to OW, but you can't get around touching in Rescue. You can give the option to opt out, but then they're also effectively opting out of certification or, in rescue, even meaningfully taking part in the training.I think it is logical that someone who is uncomfortable with being touched may also be uncomfortable making a public statement about that, particularly in a group where their peers are not opting-out.
I do a lot of yoga, possibly the furthest thing from hand-to-hand combat.
Many instructors will offer hands-on assists/adjustments. Sometimes that is very personal touching.
Now, virtually every instructor notifies the students in advance and allows people to opt-out.
Good instructors allow people to opt-out "anonymously", without letting the rest of the class know their preference. This doesn't need to be a formal thing. For example, early in a class, while students have their eyes closed or are lying face down, the instructor may say that anyone who does not want to be touched should put their hand palm-up, or move their foot off the edge or their yoga mat, etc. This action is visible to the instructor only.
A similar technique would probably be very helpful in getting consent (or allowing people to opt-out) from being touched during SCUBA instruction. Perhaps ask students to float face-down in the pool (snorkeling) and raise one hand above the water to signify that they do not want to be touched.
(And I'm just realizing this is a cave thread, so I've totally taken it off on a tangent....)