The transmission distance of underwater wireless optical communication (UWOC) is severely limited by the rapid decay of light intensity in water. Power-efficient pulse position modulation (PPM) and ultra-sensitive multi-pixel photon counter (MPPC) open the door toward designing long-reach UWOC systems. In this paper, a 46-m UWOC system based on PPM and MPPC was proposed and experimentally demonstrated with ultra-low transmitting power into the underwater channel. Clear eye diagrams without any slot error for ten different PPM signals were obtained in the 46-m experiment with data rates of Mbps level. The received optical power was as low as -39.2 dBm for the 10-MHz 4-PPM signal, when the laser worked under the stimulated state. Meanwhile, the received optical power can be reduced to -62.8 dBm, for the 5-MHz 64-PPM signal when the laser worked under the spontaneous state.