The properties of the underwater acoustic communication channel at medium range and medium frequency are characterised by analysing a set of at sea data collected in coastal environment (depth 10-40 meters, range 1-3 km, frequency 11.2 kHz or 17.5 kHz). Based on an empirical mode decomposition, we infer that the channel behaves as trend stationary random process over a few minute duration. Moreover, according to statistical tests, the channel envelope is found to be in good agreement with a Rice process whose power ratio between the mean and the scattered part is time variant and delay dependent. As a conclusion, the results of attempts to apply some approximate bounds for the capacity of WSUSS Rice channel to the measured channels are given. It is found that the channel capacity is generally close to Shannon capacity when the channel response includes a few specular paths but significantly decreases with the number of paths and the diffuseness of the channel.