The available bandwidth of the underwater acoustic channel is severely limited. Therefore, it is of interest to use modulations with high spectral efficiency, such as rotated constellations. These constellations offer better theoretical performance than conventional constellations, without reducing the spectral or energy efficiency of the system. However, in practice, when conventional equalization algorithms are employed for a system using rotated constellations, the theoretical gains are not fully achieved. To address this issue, this paper proposes a new turbo equalization algorithm that inherently considers the characteristics of rotated signals and allows for a significant gain compared to conventional methods, as demonstrated by simulations, in particular over a real underwater channel.