They won't prevent you from getting the job, the problem is you will have to be certified in whatever they are teaching. For example a NAUI instructor can teach NAUI, but he can not teach PADI classes. The basic and Advanced classes are the only ones that are universally accepted, all of the other classes seem to be agency specific.

I would perfer to learn from another agency!!! Believe me!!, but the fact that PADI is everywhere I have ever thought of going is huge!

Now as far as SCUBA instructors doing " it for thier own ego or the almighty dollar ", well what other reason is there? The reason anyone teaches is their ego. They feel like they know something and get a good feeling from sharing with others. Now as far as money is concerned, we all know that most professional divers (not commercial divers) are not making much money unless they happen to be really lucky. So that idea is down the drain, but if there was money to be made, they deserve it. A lot of hard work goes into becoming a dive instructor and should be properly compensated.