The underwater acoustic channel is time-varying with a particularly limited bandwidth. This makes reliable communications difficult to achieve and limits the information rate. The rotated constellation is an effective technique that saves both bandwidth and energy. It allows to take advantage of the channel diversity and thus to enhance performance of rotated constellations over some fading channels such as underwater acoustic channels. Nevertheless, this solution increases dramatically the complexity at the receiver side. To face this problem, this paper proposes to use a series of rotation angle α = arctan(1/ √ M), which introduces several structural properties that can be used to tremendously ease up the soft demapping process. Moreover, this solution achieves almost the same performance as the theoretically quasi-optimum Max-Log demapper with over 80% of global complexity reduction. This coded modulation has been tested successfully over several acoustic underwater channels.