In UWAC (Underwater Acoustic Communications), underwater acoustic channels change rapidly due to various environment condition. AMC (Adaptive Modulation and Coding) technique is efficient method for improving the system efficiency by changing transmission parameters according to channel conditions in underwater acoustic channels. In this paper, we propose four modes and three threshold detection algorithms to determine the transmission mode in AMC technique. In transceiver model, convolutional coding is used to free the size of the information bits and turbo equalization is applied to improve the performance. In addition, the relationship between the modes and the threshold is analyzed through underwater experiments.