In my opinion, the only specialty worth getting is EAN. You must be EAN certified to dive/rent EAN tanks. The rest, there is no prerequisite for doing any of those type of dives. Yes, knowledge is good and everything you can learn will help you be a better diver, however, unless you are financially independant, you are spending a lot of money for those specialties that could be used in better ways. If you want to learn dry suit: Get the knowledge you can get from the books and then borrow someone’s drysuit and have them show you how to use it. If you personally know an instructor, hang with them and pick up what you can about the different specialties. Personally, I just don’t see a reason to spend money on specialty courses aside from EAN.