The exploration and utilization of marine resources has promoted the rapid development of marine science and technology, which has put forward higher requirements for underwater communication technology. Long distance UWOC requires the selection of light source at the transmitter side. LDs have excellent portability and maneuverability, and have been widely used in the UWOC systems. However, its beam quality is not so good and it is difficult for modulation under high power. In recent years, vertical-external-cavity surface-emitting laser (VECSEL) has received much attention for its high output power and good beam quality. The aim of this paper is to explore the advantages of using a 490 nm blue VECSEL as the light source in UWOC, and to improve the performance of the UWOC system by the soft-decision PPM. First, the optical power attenuation coefficient of the channel is performed, and the measured c is about 0.0591 m ^-1in a 96 m tap channel. Subsequently, soft-decision and hard-decision are simulated and experimentally verified. Both simulations and experiments show that the BER can be significantly reduced with soft-decision. Afterwards, we improved the system using the soft-decision algorithm and investigated the communication performance of 64 PPM at different bandwidths by adjusting the PPM signal rate. Finally, 50 MHz was chosen as the signal rate in the experiment. Then a UWOC system is demonstrated in this paper. The transmitter side consists of a 490 nm VECSEL light source with an acousto-optic modulator (AOM). The PRBS is loaded into the AWG for digital-to-analog conversion after PPM modulation, and the analog signal is sent to the driver of the AOM for acousto-optic modulation of the incident beam. The laser is focused before entering the AOM and then collimated after exiting to reduce its divergence. The modulated laser beam passes through a distance of 96 m in the tank by using multiple mirrors on both sides of the tank. Then, the beam was focused by a lens to the APD for photoelectric conversion in the end, and the signal is processed by the MSO after data acquisition. A soft-decision algorithm is introduced to further optimize the performance of the PPM modulation. When the optical signal passes a relatively long distance of 96 m, the measured BER is as low as 1.9 × 10 ^-5. This indicates that the soft-decision PPM-based 490 nm blue VECSEL UWOC system performs very well.