The operation of a Lamb wave device, using an interdigital transducer and a piezoelectric ceramic plate with spatially varying thickness, is investigated both theoretically and experimentally to understand the characteristics of ultrasound radiation into water that accompanies Lamb wave propagation. A new calculation model for Lamb wave propagation in a water-loaded inhomogeneous plate is established and the Lamb wave velocities as well as the radiation efficiencies of the compressional waves in water are presented. There is small velocity gap deriving from the inhomogeneity, but the radiation efficiency is much higher or lower than that in the case of a flat plate, depending on whether the Lamb wave propagates to the thinner part or the thicker part of the plate. The amplitude, radiation angle, and the focusing characteristics of the compressional waves radiated in two directions are observed to be compared with the numerical results, showing the asymmetrical relations between the two ultrasound beams. The frequency dependence of the radiation is also discussed. A transducer in this configuration, with the advantage of frequency-dependent radiation, is effective for applications such as nondestructive evaluation and medical acoustics.