The invention is to provide an underwater communication device and an underwater communication method that can be applied to mobile communication in water. An underwater communication device 10 includes: a transmitter 12 configured to transmit an electric signal; and a receiver 14 arranged away from the transmitter 12 via water and configured to receive the electric signal, in which the receiver 14 includes an field effect transistor 23 having a channel region 39 provided at a position in contact with the water, and a drive circuit 26 configured to generate a potential difference between a source region 31 and a drain region 33 of the field effect transistor 23.