While you can get basic scuba lessons at a surf shop or while on a cruise, to become a scuba instructor you will need dedication to hard physical training for long months, the money to investment in swimming equipment, and the ability to travel to various bodies of water so as to train under various weather conditions. You'll have to get your scuba certification from a certified training school. In many places even to apply to become a scuba instructor requires that you be over 18. While all certification programs to become a scuba instructor aren't exactly the same, they all have similar basic requirements. More than anything else, you'll need to log hundreds of hours diving under all sorts of conditions.
To start with, before you can become a scuba instructor you'll need "entry level" scuba diving lessons. This very basic course can often be taken at the aforementioned surf shops or on cruises. In many cases, they provide equipment to use during the duration of the course, and can have you in the water in as little as two days. A short written test is usually required, and you'll practice diving in a lake, swimming pool, or sometimes the ocean.
This is only the first step when you become a scuba instructor, however. You may begin your scuba diving lessons in a heated pool, but to become a scuba instructor you will need to dive in open water. As an example, the Scuba Educators International (SEI) diving school certification calls for 10 hours each of diving to more than 80 feet (24.384 m) below the water. It also requires diving in a wetsuit, diving in little more than your skin, driving in salt water and fresh water, water where there's low visibility, and diving at night. The Professional Association of Diving Instructors (PADI) course requires that you swim 2,625 feet (800 m) with only the help of a snorkel and fins.
As you can see, to become a scuba instructor you will need to either be in good physical condition or train until you become so, and many schools require you to have a certificate from your doctor certifying that you are in good health. In addition to the physical requirements, in order to assure that you know the concepts behind dive theory, you'll have to take extensive written tests or even give lectures on becoming a scuba instructor. Many scuba certification courses ask that you assist in teaching others for a certain amount of hours before you qualify to become a scuba instructor. Finally, you'll need to be certified in first aid and CPR in order to be prepared for emergencies and keep your students safe.