Underwater wireless optical communication (UWOC) is a key technology for supporting information transmission in the Internet of Underwater Things (IoUT). Recently, semantic communication has been introduced into UWOC systems to enhance transmission efficiency and robustness. However, existing underwater wireless optical semantic communication (UWOSC) systems overlook the bandwidth limitation imposed by practical optoelectronic devices, which hinders high-speed semantic information transmission. In this paper, we introduce orthogonal frequency division multiplexing (OFDM) modulation into the UWOSC system for the first time, and design a novel OFDM-based UWOSC system with symbol and power loading (SPL). In the proposed system, shifted-window-based hierarchical vision transformer blocks are employed to extract semantic features and encode them into frequency-domain baseband symbols for OFDM transmission. Furthermore, an SPL scheme, comprising an entropy model-driven symbol reordering strategy and a lightweight power allocation network, is designed to adaptively allocate subcarriers and power based on semantic importance and channel conditions. Experiments conducted on an emulated UWOC platform demonstrate that the proposed OFDM-based UWOSC system achieves superior performance compared with baseline schemes under high transmission bandwidth scenarios. Moreover, the introduction of the SPL module further improves semantic spectrum efficiency, effectively mitigating the adverse impact of low-pass characteristic inherent in bandwidth-limited UWOSC systems. These results demonstrate that the proposed system has strong potential to enable high-speed semantic communication in the IoUT.

Recently, underwater wireless optical communication (UWOC) has emerged to support the high data rate requirements of oceanic exploration. In this paper, we propose an accurate and closed-form UWOC channel model to understand the effects of dynamic ocean environment on optical signal propagation. The model takes into account the impairments induced by oceanic path-loss, oceanic turbulence, pointing error loss and link interruption due to angle-of-arrival (AoA) fluctuations jointly. We further derive analytical expressions for various outage performance metrics. To boost the system robustness to dynamic ocean environment, we design a probabilistic shaping (PS)-based strategy with unipolar pulse amplitude modulation (PAM), which maximizes the ergodic constellation constrained capacity. Furthermore, considering the limitation of computational resources in real ocean environment, we simplify the PS-based scheme to alleviate the problem. Numerical results verify the accuracy of the proposed channel model and the outage performance analysis. Moreover, the simplified PS-based unipolar M-PAM scheme is validated to be a promising solution for the development and deployment of high speed adaptive UWOC systems.