Thanks for the replies so far but can people please answer my specific questions. Please don’t bother bringing HSE again, I will be doing the scuba level 1 course in the future anyway
Check the Commercial Diver forum: Commercial Divers. There have been posts over the years from people asking about starting a small hull cleaning business.
Maybe diving for small articles dropped in a marina is in a different category but hull cleaning, even on a small scale, apparently deserves the considerations (safety, legal, etc.) given to commercial diving.