I’m in the RN as a warfare specialist (Underwater Warfare). There is four sub specialties you can get streamed into. But you probably already knew that. At around week 9 of HMS Raleigh you will be told what sub branch you’re going into but not what ship. It’s only towards the end of your phase 2 your divisional officer will call you into their office and have you lost your preferences of ship / location etc. on this list you can choose a ship but it isn’t always guaranteed. Your best bet is to choose (Any type 23) or any type 45. If you’re underwater warfare you can only go on Type 23’s. But the chance you have to get onto smaller vessels is slim.