We describe herein highly effective asymmetric hydrogenation of quinolines in undegassed water catalyzed by chiral cationic Ru-diamine complexes for the first time. This facile and green protocol is applicable to the scaled-up synthesis of 6-fluoro-2-methyl-1,2,3,4-tetrahydroquinoline, a key intermediate for the preparation of the antibacterial agent (S)-flumequine, with 98% ee.