As an altative to satellite com-munications, multi-hop relay networks can be deployed for maritime long-distance communications. Distinct from terrestrial en-vironment, marine radio signals are affected by many factors, e.g., weather conditions, evaporation ducting, and ship rocking caused by waves. To ensure the data transmission reli-ability, the block Markov superposition trans-mission (BMST) codes, which are easily con-figurable and have predictable performance, are applied in this study. Meanwhile, the phys-ical-layer network coding (PNC) scheme with spatial modulation (SM) is adopted to improve the spectrum utilization. For the BMST-SM-PNC system, we propose an iterative algo-rithm, which utilizes the channel observations and the a priori information from BMST de-coder, to compute the soft information corre-sponding to the XORed bits constructed by the relay node. The results indicate that the pro-posed scheme outperforms the eonvolutional coded SM-PNC over fast-fading Rician chan-nels. Especially, the performance can be easily improved in high spatial correlation maritime channel by increasing the memory m.