JOINT BASE PEARL HARBOR-HICKAM, Hawaii—Inside the flooded ballast tanks of a Virginia-class submarine, where there is no natural light and spaces are tighter than shoulder-width, Royal Australian Navy divers are learning how to sustain one of the world’s most advanced undersea platforms. Although the work is gritty, technical, and claustrophobic, it represents one of the clearest signs yet that AUKUS—the enhanced security partnership between Australia, the U.S. and the U.K.—is progressing and delivering results.