If you want to hire a scuba instructor, you need to establish his or her technical credentials. You should be relaxed and comfortable with your teacher's style of instruction and be able to trust his or her communication skills in underwater surroundings.

First of all, locate potential instructors. Get references from a qualified dive shop; most work with or recommend particular instructors. Consult with the shop why it chooses that instructor. The best choice could be an instructor that is affiliated with a reputable dive shop.  

Inquire about the instructor's credentials. Many of the U.S. and international training organizations certify instructors. The Professional Association of Diving Instructors is one of the largest. Some states have regulations for U.S. diving instructors, but there are no actually federal regulations. A well-informed instructor should know these rules, if they are applicable, and should have up-to-date certification.
  
Ask about the classes and the student-to-instructor ratio. Inquire where the pool is located.

Consider for how long the certification program runs and how the sessions are scheduled. A conscientious instructor gives classroom lectures before taking to the water. There should be sufficient time for personal attention from the instructor and questions from the students. Inquire how you can make up missed classes if necessary.  

Check out the equipment provided to students. See whether there is dry, cracked rubber on suits, fins and hoses and check for condensation inside gauges.

Ask about open-water dive locations. There should be rescue personnel and medical facilities available. In case you will be taken to a dive site by boat, check whether that boat is seaworthy and equipped with a radio, fire extinguisher and life jackets for all passengers.