IMO....
Mentoring is an explicit relationship between individuals to transfer knowledge during the actual practice of an art.
In other words, if I was to hire a junior engineer, I would partner them with a senior engineer and tell the senior engineer to mentor the junior. The junior would be expected to do her own work, to accomplish goals, etc... so would the senior/mentor... but if the mentor observed the junior doing something sub-optimal, or failing to do something that should have been done, the mentor would be expected to step in and lend advice and/or assistance with the purpose of bringing the junior up to speed. Both engineers would be paid for their work...perhaps not the same amount, but paid.
Instruction is an explicit relationship between individuals to transfer knowledge OUTSIDE the actual practice of an art.
In other words, if the junior in the previous example decided that the engineering work she was doing required a better grasp of microcontroller development practices, she could seek instruction in that subject. The instruction would take place outside of her routine work, and would involve exercises intended to develop knowledge and skill rather than intended to produce the normal results of the practice of her art (commercial engineering).
It is entirely possible that an instructor and a mentor are the same person, but the methods are very different.
I have lived the scenario of being given a junior engineer to mentor, and realizing that the person lacked the basic knowledge necessary to do even rudimentary work in our field. I took the situation to my manager, who basically said, "deal with it," so I fell into instructor mode, assigning training exercises and going through a basic curriculum of study designed to bring him up to speed and make him productive. For awhile I instructed instead of mentored. Eventually he learned enough that I could drop back to the mentor role again. To me it was very clear when mentoring ended and instruction began - when his best effort was insufficient to the actual practice of the art we were paid to practice AND the decision was made to train instead of firing him.
I don't think the mentor role can really exist outside of some framework where the mentor is obligated to assist the mentoree. That framework may be social ("pay it forward"), commercial (paid mentoring), familial (learning to work on computers at your mother's elbow), educational (university mentoring), business (my example above), military, etc.... but there seems to be a need for some external force or obligation beyond mutual interest in the practice of an art or skill. Just diving with someone, or playing music with them, or designing rocket ships with them, doesn't make them your mentor or vice versa.
Of course all of the above applies to diving or any other activity.
