I would not pay the money to a "wannabe professional UW photographer" for one-to-one teaching (as many offering such courses are). Instead read the book by Martin Edge (cited above), even better to my opinion (didactically excellent) is "Underwater Photography Masterclass" by Alex Mustard.
Afterwards a course within a small group and learn from a real master, who is very patient and didactically skilled (I made two workshops so far and can only recommend his workshops):
WORKSHOPS – ALEX MUSTARD
Wolfgang