In August 1808, owing to the Duke of Kent's recommendation, he was made aide-de-camp to Brigadier Charles William Stewart (afterwards third Marquis of Londonderry). He went with him to Portugal, and served in Moore's Corunna campaign. Sent out to obtain intelligence of the French movements in December, he bought from the Spaniards at Valdestillas an intercepted French despatch from Berthier to Soult, which gave Moore most important information, and at once altered his plans. Waters was promoted major on 16 February 1809, and was attached to the Portuguese army (with the local rank of lieutenant-colonel), but employed on intelligence duties. Wellington wrote of him on 26 October, when he was going home for a time with Stewart: