Merrill Lynch on Thursday said it cut its rating on oil drilling companies Rowan Cos Inc., ENSCO International Inc. and Global Marine Inc., citing slack demand in the Gulf of Mexico drilling market. Merrill said that it previously believed drilling demand would improve by the late part of 2001, but it now looks as though that will not occur until early 2002. Shares of Global Marine were down $0.33 cents at $15.30. ENSCO was down $0.27 cents at $18.63, while Rowan was down $0.06 at $16.95 on the New York Stock Exchange.