I dont really agree with you that outside of photography pro is a measurement of quality. In my perception pro means they made it their profession. A pro soccer player (from europe after all) gets paid to play soccer all day. The result of this is that they become better at it than an amateur that plays on sunday afternoon for fun. So the effect is increased quality, but the cause is daily rigorous (paid) training.
Photography, and especially underwater photography, has a huge influx of affluent people that can afford to practice a lot (and thus increase their quality) without making it their profession. (in most cases because being a heart surgeon is already a fulltime profession). Like you say, and has been mentioned in this thread, their output quality can often compete with people who made this their profession. Just look at some of the competitions and you can see this. Any field where the quality difference between paid and non/low-paid performers is small, is bound to have some issues regarding the delimitation and definition between the two.
The two definitions for uw pro photographer that tend to get mixed together to form an unquantifiable mix is "someone that is really good at creating underwater images" and "a (paid) underwater industry professional". Some people lean more to the former, some more to the latter.