The direct-adaptation based turbo equalizer (DA-TEQ) has been widely studied for underwater acoustic communications due to its decent performance and simple implementation. However, there are still some inherent problems that limit its practical application, such as slow convergence rate and error propagation effect. In this paper, a direct-adaptation based bidirectional turbo equalizer (DA-BTEQ) is proposed for underwater acoustic communications. The proposed scheme incorporates a forward DA-TEQ with a backward DA-TEQ to exploit bidirectional diversity gain and combat error propagation, thereby enabling faster convergence rate and better symbol detection performance. A general symbol combining scheme, which is suitable for turbo equalizers with high-order modulations, is derived by using the minimum mean square error criterion. The proposed scheme has been tested by the undersea trial data collected in an experiment conducted at the coast of Jiaozhou Bay in March 2017. The results demonstrate that the DA-BTEQ is effective against error propagation and clearly outperforms the traditional single-direction DA-TEQ for both single-input multiple-output and single-input single-output systems.