<p>Designing and developing a routing protocol for underwater acoustic sensor networks (UWASNs) has always been challenging due to the unpredictable and harsh nature of the underwater environment. Opportunistic routing for UWASNs can provide a way to enhance network performance, using the broadcast nature of the acoustic wireless medium. However, the majority of opportunistic routing protocols only focus on network lifetime, hence the communication link often suffers from propagation delay and excessive packet overhead. In this article, the Hybrid Message Encoding based Routing Protocol (HMERP) is proposed, which takes advantage of the wireless acoustic broadcast medium to cope with the dynamic nature of the UWASNs. HMERP incorporates a novel message encoding algorithm to optimize inter-node packet transmission in a dynamic underwater environment. Additionally, a hybrid routing framework is proposed to integrate deterministic routing with opportunistic listening. To enhance network throughput, the proposed routing protocol proactively initiates the path-switching mechanism and establishes an optimized routing path with minimal hop value towards the sink node. Thorough tests of the simulated network show that the proposed routing protocol reduces the propagation time, energy consumption of the nodes, and increases the packet delivery ratio compared to classical opportunistic-based routing protocols.</p>