I find it very difficult to define a pro. For me personally photography will never be a substantial fraction of my income.
When organizers try to put some kind of 'amateur' or 'pro' stamp on their competitions I think what they really mean to say is that they are looking for specific skill levels, assuming that amateur/pro is somewhat similar to 'less skill/more skill'. Obviously if your profession is photography, you can infer that you'll probably be good at it or you would be looking for a different career.
But we all know that especially digital photography, and in our case underwater digital photography, attracts a lot of people with perfectly fine successful careers in a different field, who manage to become quite skillful in photography. You see it right here on wetpixel where there are a lot of very capable photographers that would not have to be ashamed of themselves if you put them next to any pro. Some manage to eventually make a career out of it and can quit their day job, but most can't or more likely, won't. It doesn't make them any less skillful.
So when competitions try to avoid "pros" I think what they really try to avoid is the top tier photographers, not necessarily only the ones that make a full living out of it. For this reason I usually try to read between the lines of competitions are see who they are trying to target.
I think it makes more sense for competitions to just have a 'placed in a competitions before' rule and forget about the pro/amateur. No self respecting real pro (and here I do mean the ones that make a full living) will enter a competition that targets people 'that have never placed in a competition before'. Any that do (and I do know an example or 2) imho just ridicules themselves forever.
Cor