Abstract:  A  full-depth underwater acoustic modem was designed to accomplish the high rate acoustic communication at any area of the oceans. The modem was equipped with a D-type power amplifier that was designed to output 200 watt electrical power. Three hydrophone was used at receiving-end to demodulate the acoustic signal, which formed a multi-channel space diversity receiver. The maximum communication distance of the underwater acoustic modem was 12 kilometers, and a single-carrier QPSK coherent modulation was used to transmit information at 6000 bps data rate. A bi-directional turbo equalizer was used to demodulate the information and fulfill the channel-decoding performance. The filter coefficients of the equalizer was adapted by an improved proportionate normalized minimum symbol error rate rule. After the prototype of the modem was finished, a sea trial was performed at a 10500 meters depth in the Mariana Trench on December 12, 2018. Tens of communication data groups was sent from the bottom of the sea. The total bit error rate of the data was 10^-5 at the receiving end.