If you are talking about being a professional dive guide (as in being paid), I am afraid it is a bit more complicated than presented.
To begin with, to get paid, you need to get someone to pay you. Unless you want to freelance and get next to no business, you will have to be employed by an organization that leads dives. They will want to be sure you have the expertise to lead the dives without incident. They will also want to be sure you are insured. (OK, we see there is an incredibly unusual exception in the San Diego accident story, but that is a really unusual case.) That will mean having at least a Dive Master/ Dive Con type of certification, which is well beyond Rescue Diver. Those ratings are professional, and in order to keep them current, you will need to maintain professional liability insurance.
Leading dives is not all you will do for an organization like this. You will be expected to do a lot of other jobs that will help keep the organization in business. That includes menial labor, like hauling tanks and mopping decks. The more jobs you can do, the more employable you will be. In major resort areas, most dive guides are actually instructors. These people are more employable because they can instruct as well as lead dives.
There are many, many people with these qualifications looking for the available jobs, so competition is fierce.
I would be shocked if any major resort area dive operator employs dive guides with only Rescue Diver certification.
