I can recommend David J P Mason, 'Roman Britain and the Roman Navy' (0-7524-2541-2) £17.99/$29.99, another useful book from Tempus. Indeed the only other book on the Roman Navy that I know is Chester Starr, 'The Roman Imperial Navy', first published in 1941, and now very expensive. Mason is very good at describing the various kinds of ships, and the organisation of the navy, and, while there is scant archaeological evidence, his conclusions are plausible and complement the usual accounts.