Answer:
Most profitable one is to own a dive shop and training center.
Taking 10 students on OPEN WATER at £300 a go makes you some decent money. add to that the income from gear sales, and other training courses you sell.
If you dont want to do the retail side of it, then work hard, become a Master Instructor, and get reccomended for a COURSE DIRECTOR PROGRAM.
These are held twice a year and only a select few get accepted and pass.
A Course Director is the Highest of the Highest Ratings in scuba (under Padi scheme) and you will get to travel the world and name your own price!