Depends what your work is requiring you to do - if you are going to be floating about in 10 metres of open water (ie not going inside anything) looking at stuff and taking notes about how many rivets are in a particular underwater pole, then probably recreational training and some diving experience is all you need. Given that you would be, by default, task loading yourself, I would recommend you take yourself as far as Rescue Diver (or agency equivalent) and get a fair few dives under your weightbelt.
There's no cert that I'm aware of in the recreational world that will specifically qualify you to carry out inspections, and I'm not certain that any broad-spectrum training in any form of diving would qualify you to do that - although persons such as Thalassamania above will know a lot more about that than I.
If you're planning on actually participating in underwater construction, or the inspections require hands on work and tool manipulation under water then for sure you will need commercial training. Some of these will require that you hold a professional level certification from the recreational diving world before you commence training. HSE Commercial diving in the UK, for example, requires that you are a PADI Divemaster or equivalent prior to commercial certifications, even though the two fields are often unrelated.
You will of course get better information from prospective employers as to what certifications they would require of their diving inspectors, but hope that helps a bit.
Cheers
C.