Shallow earthquakes occurring in the southern part of the Tyrrhenian basin excite a long-duration low-frequency coda that propagates efficiently along paths crossing the Tyrrhenian Sea. Seismograms recorded at regional distances in the Italian peninsula are analysed in the frequency-time domain and compared with synthetics computed in simple vertically varying media. Based on results of our modeling, we interpret the coda as short-period (5--10 s) suboceanic Rayleigh waves. Their large amplitude and long duration, that we emphasize here, are explained as due to the influence of the Tyrrhenian water layer over a fast seafloor with little S-wave attenuation (QS > 300). A satisfactory fit of amplitude and duration of the observed waveforms is obtained assuming a young oceanic model with an average water column of 1 to 2 km and relatively high S-wave velocity (2.5 km/s) in the shallow crust.