i just completed an open water class. that seems to go against everything my instructors taught me.
For the book learning portion of the class, online is probably fine. The tests of course have to be done proctored in some way. I think the reason my students do well in my online courses (I teach information systems not scuba diving) is because I provide video lectures and hands on things to do to get comfortable with the material. The nice thing about that is that auditory learners can listen to the material several times so that they get a better handle on it. Some people just don't do well with books.
Basically, for the "brain" part of any class I feel that delivering the material in as many ways as possible is best.
If you are interested to find out how best you learn go to VARK -- A Guide to Learning Styles.
I think all that any instructor wants is for students to get a handle on material.
SMW