There's no 'Cadetship', as such. However, take a look through this website and you'll get a good idea of the qualifications required to assume command of a Windfarm Support Vessel.
Workboat Deck Courses
Please note that I have absolutely no association with this organisation, am not in a position to recommend them (because I have no personal experience) and if you search on Google you'll find there are other organisations offering these courses nationwide.
If you're looking to commence work with one of the workboat operators as a deckhand you'll require as an absolute minimum the STCW95 Sea Survival ticket and an ENG1 medical certificate.
Pieces of paper are necessary, of course, but once you have these it all boils down to personal attributes; attitude and experience are everything. Many people simply cannot cope with living/working in close proximity to (say) 5 other men on a 24 metre boat that's working 24/7 for a week or two, 50 miles offshore in sometimes 'challenging' conditions.
Excellent crew are always in demand, well looked after and remunerated.