A Hierarchical AI-Driven Hybrid Communication Framework for Underwater Swarm Coordination: Achieving Sub-Microsecond Synchronization and Robust Cooperative MIMO Performance. - Mandana Mahgoli; Ian F. Akyildiz; Kristinn Andersen; Saemundur E. Thorsteinsson; Magnús Örn Úlfarsson (IEEE Internet Things J., 2026)