When I was "in" the initials RA stood for "Ration Allowance" i.e. you were living off your "ship" (which could be a shore establishment) which meant that the Navy paid you a daily allowance in addition to your pay to feed yourself.

Those who normally lived off their ship were submariners when in port and the Royal Navy's Bomb and Mine Disposal Diving Teams who had the job of surveying wrecks whose cargoes are dangerous (USS Richard Montgomery in the Thames Estuary off Ramsgate is probably the best known example).