Waving two arms in the form of an X might be the correct way, but most of the general sailing public have no idea what it means. I was once some 4 miles offshore in the English Channel in an 18 foot open sailing boat with engine failure. Not much danger, but it was a flat calm, so sails no good. Was passed by multiple other boats who either ignored my signal, or just waved back. Had to call the RNLI for a tow, much to my embarrassment.