Really... being a scuba instructor is something you for for the love of the diving, and not for the salary.
You won't make much money as a scuba instructor. IF you are successful, it will be enough money to live on, but you won't be living any sort of bling lifestyle...
In truth, a very tiny percentage of scuba instructors manage to earn sufficient money to stay with diving as a full-time, life-long career. Many do it for a few years, as a sabbatical etc, before going into a mainstream job because they need more income. Many just do it part-time for extra cash and because they love diving and teaching.