Physical layer security techniques have emerged as promising candidates in order to achieve perfect secrecy in wireless communication systems. In this framework, an approach is proposed to enhance secrecy, taking advantage of topological and mobility asymmetries between a legitimate user and potential eavesdroppers. As a specific example, an Alamouti Orthogonal Frequency Division Multiplexing (OFDM) system with distributed mobile transmitters is investigated for underwater acoustic and radio frequency applications. The mobile transmitters independently emit pre-equalized - in terms of Doppler shifts - Alamouti OFDM symbols. As a result, the legitimate receiver effortlessly receives Doppler shift free copies of the OFDM symbols with standard diversity gains. On the contrary, eavesdroppers experience diversity gain compression and cross-symbol interference. A positive ergodic secrecy capacity is achievable even in the absence of an SNR advantage at the legitimate user.