Can I work in Australia as a Diving Instructor if I have already had a working holiday visa?
I doubt it. While you now have a skill it has to be on the SOL list of professions for migration. You need to check out www.immi.gov.au to see if diving instructor is there. As there are only a very small number of places that you dive in Australia (mostly north Qld and northern WA) it is not a widely required profession. Professions are only listed on the SOL if there are no where near enough Australians trained in those professions. Anything to do with the Great Barrier Reef is not likely to be on that list. As you are not eligible for a second WHV you can only come on a migration visa or marry an Australian.