The ocean Internet of Things (IoT) has played a crucial role in ocean observation and sea area monitoring applications. The complex and dynamic marine environment leads to time-varying characteristics of ocean network links, posing significant challenges to the ocean IoT. Therefore, a rate-adaptive cross-layer opportunistic transmission (RA-CLOT) protocol is proposed, which integrates physical layer adaptive data rate, MAC layer carrier sense multiple access, and network layer opportunistic routing together. This protocol opportunistically selects the best forwarders through a three-way handshake mechanism, and adjusts the transmission rate based on the link dynamic signal-to-noise ratio, aiming to enhance the robustness of the ocean IoT in the complex and dynamic marine environment. Experimental results from land and lake trials demonstrate that the proposed protocol achieves a high packet delivery ratio with reduced energy consumption and latency under a complex and dynamic network environment.