I think the name Sharkey means 'old man'. I'd have no idea though unless I reread The Scouring of The Shire.
Saruman didn't choose the name Sharkey himself; this was a nickname his servants used for him in Isengard. He just found it amusing.
Because he was probably the only one in the nation who knew what a shark was?
Except that the name Sharkey had nothing to do with fish (in-story). It was Orkish for 'old man'. ShireofMiddleEarth remembered that correctly.