A Review of Energy-Efficient Routing Protocols for Underwater Acoustic Wireless Sensor Networks

Underwater acoustic sensor networks (UASNs) are key enabling technology for the Internet of Underwater Things (IoUT), supporting environmental monitoring, offshore exploration, disaster warning and surveillance. Existing routing approaches include depth- and hop-by-hop schemes, geographic/vector-based forwarding, clustering, opportunistic and cross-layer routing, and more recent reinforcement-learning and multi-modal designs. Depth-based methods are simple and localization-free but can create redundant forwarding and energy hotspots near sinks; geographic methods provide directional forwarding but depend on difficult underwater localization; clustering improves scalability and aggregation efficiency but incurs cluster-head and re-clustering overhead; opportunistic methods improve robustness to unreliable links but add coordination cost; and learning-based methods adapt to dynamic channels but introduce computation, memory and convergence overhead. This review develops a two-axis taxonomy that separates localization requirements from forwarding strategy, critically compares representative energy-efficient protocols, links routing choices to security and privacy risks, and identifies open questions in secure learning, energy harvesting and acoustic-optical-RF modality switching. The analysis emphasizes the trade-off among network lifetime, packet delivery reliability, delay, control overhead, scalability and security, and it provides a protocol-suitability matrix for selecting routing families for practical UASNs.