As a professional artist, I remember people telling me as a kid to become a wealthy doctor & captain of industry, & when I own the paint company, I can keep a little for my own art. Most of these well-meaning people were pro artists themselves & had suffered the ups & downs of their careers. I would hear none of that. I got on a bus to California early one morning. I never looked back.
I am not rich, & am looking at the possible need for a career change at age 37. Nonetheless, I did what I wanted, made some $, had adventures, escaped a miserable small town, met my husband & other exciting creative people, and I do not envy my sisters who stayed behind with their educations & jobs in retail. I caution young people only because I do not know what their expectations are (getting rich, famous, finding a sexy model to marry them, etc.) But if you are going to persue a life of adventure with low pay, your young years are the best time to do it. Travel works best before you are married with kids. You can always go to university later when u r tired of the instability. But you only have your youth for a short period of time, & if you think travel & excitement are for you, there are jobs out there & someone has to do them. Research which cruise lines hire dive instructors, check out resorts on the internet. Find out what Instructor training Courses will give you a real advantage, not just be easy to pass. Cultivate knowledge about the ocean, & use your classes at school to investigate oceanography & marine biology. Stay intellectually curious; you will always need an active mind.
The older & wiser people who caution you are thinking from a different perspective; we are more concerned with mortgage payments, car repairs, health insurance, & other boring grown-up stuff. It is good to have a trade other than diving (ie: something that does not take too long to learn) while you are deciding what to do with your future. Who is to say that you won't get in touch with your spiritual side on Grand Cayman, hear about a new career field in Belize? If you are still trying to find out who "you" are, you just need to have experiences. Frankly, I think you are more likely to stay out of booze & drugs as a diving instructor than if you rush into college or the military. You will have responsibilities, & you can't drink & dive. You will have enough money for a simple life, & when you are young, that's just fine. If you stay in England, your first job will probably be working for a jerk anyway, so why not a jerk with a dive shop or boat?
I think "follow your dreams" does not mean you are being foolish. Not everyone is great in school; not everyone is interested in book-learning at this stage of life. If you "waste" a year teaching diving in Fiji & find out it is not for you, it is better than following a career you hate & saying when u r 60, "Gee, I wonder what life would have been like if I just tried teaching diving?"