Considering the attenuation characteristics of the seawater channel in underwater wireless optical communication(UWOC) and the influence of system self-noise on the modulation performance, an improved Gardner timing synchronization algorithm is proposed,and an UWOC system model is established based on offset quadrature phase shift keying(OQPSK) modulation, then the influence of water types and transmission distance on the bit error rate(BER) performance in the OQPSK modulation system is simulated and analyzed. The simulation results show that compared with quadrature amplitude modulation(QAM), quadrature differential phase shift keying(QDPSK) and on off keying(OOK) modulation systems, when the BER of the system reaches 10-3, OQPSK modulation can obtain the gains of about 2.2, 4.4 and 6.2 dB respectively. And compared with OOK modulation, OQPSK modulation can obtain a distance gain of 2.3-8.2 m. In addition, in the underwater channel with large attenuation, the proposed Gardner algorithm can improve the synchronization performance of the OQPSK modulation signal, and compared with the traditional algorithm, the bit error rate of the proposed algorithm is significantly reduced, and a gain of about 12.5-14.2 dB can be obtained.