My Fair Lady was a play before it was a film, of course. The Water in Majorca is a parody of the scene in which Henry Higgins teaches Elyza Doolittle how to speak properly with the phrase, 'The rain in Spain falls mainly on the plain'.
My Fair Lady was not a play, although it was a stage musical. It's loosely based on a play, Pygmalion, by George Bernard Shaw.There is no line in the play or musical about water and Majorca; as Waldo points out, it was a parody. The actor in the ad was Brian Pringle but I'm pretty sure the actress wasn't Lorraine Chase.