Synchronization is a critical operation in an underwater acoustic data communication receiver. This paper proposes a comparison of two digital NDA (non data aided) timing recovery schemes, referenced in literature as the Gardner, and the Oerder and Meyr algorithms. We consider the context of a single QPSK carrier continuous transmission, where a timing shift has to be estimated continuously in order to track the optimum sample time. Simulations processed on real data taken from sea trials, collected by the GESMA in collaboration with SERCEL and ENST Bretagne, reveal that a large Doppler shift or a large jitter on the timing shift estimation can introduce cycle slips in the clock synchronizer, which generate a large burst of errors in the data receiver. These perturbations can dramatically affect the global performance of the transmission system.