At that time H.M.S. Pembroke I wasn't an actual ship, it was a Royal Naval shore base, in this case a barracks and gunnery school, at Chatham in Kent.
http://en.wikipedia.org/wiki/Drill_Hall_Library#History_of_H...Apparently the Royal Navy men and five civilians were being carried as passengers, the USS Tampa was heading for port in Milford Haven when she was sunk.
She had just left an inbound convoy which she had been escorting to Britain, my guess is that the Royal navy sailors might have been signallers or D.A.M.S. gunners, i.e. they were manning the defensive guns on board the merchant ships and were no longer needed on board those ships once the voyage was complete.
D.A.M.S. gunners were administratively posted to i.e. were on the strength of a shore establishment which dealt with their admin and their postings to the various convoys and merchant ships that they were required to serve on.
Your relative was a stoker and he was attached to HMS Pembroke II which was the Royal Naval Air Station at Eastchurch, at Sheppey in Kent.