Jen,
AB = 'Able Seaman'. It's the next step up from 'Ordinary Seaman'.
I also note that Samuel served in HMS Cressy. There is a notorious naval incident involving the German U-Boat 'U9' and Cressy, together with HMS Aboukir and HMS Hogue, in which you may be interested. Just do a general Google and you should easily find out all about it.
My own Granddad was almost killed by this U-Boat crew whilst serving aboard HMS Theseus, in 1914. Fortunately the torpedo missed his ship, but sadly sunk HMS Hawke which was close by.
The Cressy incident led to the Navy adopting a policy of forbidding vessels from picking up survivors of U-Boat attacks in case they fell victim to attack as well.
PJ