Yes, you will be getting lots of replies in which people tell you that you will make very little money. Phrased in a not too polite fashion, the reason that scuba instructors are not paid very much is because they are worth very little.
One problem is, there are plenty of people willing to work as scuba instructors for next to nothing.
A bigger problem is virtually none of these people have any idea what the job is about, indeed even what business they are entering and most are either incapable or unwilling to do the necessary work.
Since you have eight years to prepare, here are some things you might want to do;
Join an acting group and get yourself on stage, a lot. Get comfortable speaking and performing in front of a crowd.
Join Amway, listen to all their tapes on sales, internalize it and practice the plan. It wont make you any friends, but there are no better and certainly no cheaper courses of study on sales techniques.
Join a gym and get yourself into shape, really good shape. Concentrate on back and shoulders.
Learn and practice mechanical skills, diesel and HP compressors are a must, outboard knowledge is handy.
Realize that no one is going to pay you lots of money to go diving and have fun on your own. You get paid on your ability to fill the boat, fill the classes and encourage people to return to dive or learn from you. You will also have to keep up with all the work. You will be expected to keep the boat clean and yes this includes the head (toilet for you land lubbers). You will be expected to load and unload the boat. This typically means 2000 pounds of stuff will have to be moved on and off the boat, two to three times a day. You will probably be expected to help gear people up, which means dead lifting 40-70 pounds of weight to chin height twelve times per trip, 2-4 times a day (repeat for lowering).
You will have to perform some version of a resort course endlessly. The beginners never get smarter or more skilled. Make peace with that now.
You will also have to get along with the owner of the shop, who is probably a very difficult personality and unless it is a new shop, is on the downhill slide of going out of business.
If you can do all this, you still wont make a lot, but you might be able to almost double your retirement income.