In this work, an off-grid hydrogen production system powered by marine current energy was studied, which employed a horizontal axis marine current turbine (HAMCT) and a polymer electrolyte membrane (PEM) electrolyzer fed with ultrapure water. The fluid kinetic energy of the marine current will be captured by the turbine and finally stored as hydrogen energy through the electrolysis reaction in the electrolyzer. It is important to fully understand the characteristics of the electrolyzer for the stable and efficient operation of the system. Here, a dynamic model of PEM electrolyzers was developed, which is based on the Hammerstein structure. The particle swarm optimization (PSO) method and the least squares method were used to fit the static part and the dynamic part of the model, respectively. The experimental validation shows enough precision for engineering applications and the ability to characterize the transient behavior of the electrolyzer. Faraday’s efficiency of the stack was modeled using an empirical formula. The simulation of the proposed system was then carried out using the measured current velocity data as input. The results demonstrate that the system achieved the designed operating performance with the power coefficient of 0.42 and the estimated average energy conversion efficiency from marine current-to-hydrogen of 16.4%. Then, the sea trial was conducted in Zhoushan Archipelago. The power coefficient and the average energy conversion efficiency were found to be 0.35 and 11.9% respectively, with a decrease compared to the simulated results, which was attributed to the idealization of the simulation model and the degradation of the PEM electrolyzer. The performance degradation of the PEM electrolyzer throughout experiments and its effects were discussed. The principle and feasibility of the marine current-hydrogen system were successfully demonstrated.