Abstract: With the evolution of cross-domain unmanned systems, including unmanned aerial vehicles (UAVs), unmanned surface vehicles (USVs), and unmanned underwater vehicles (UUVs), from single-platform employment to mission-chain coordination, the close-in protection of ship formations is increasingly challenged by asymmetric threats characterized by low observability, multi-directional approach, low-cost attrition, and saturation penetration. Accordingly, the focus of protection has shifted from intercepting individual targets to mission-chain identification, degradation, disruption, strike-effect assessment, and terminal mission sustainment. This paper reviews the development and key technologies of maritime multi-domain counter-unmanned systems (C-UxS) protection under confrontation scenarios between ship formations and cross-domain unmanned systems. The discussion focuses on small unmanned threats operating within close-in line-of-sight ranges in low-altitude, sea-skimming, surface, and shallow-water spaces, including small UAVs, sea-skimming loitering munitions, small USVs, small/micro UUVs, divers, and suspicious underwater devices. First, the types of maritime unmanned systems, layered operational scenarios, and mission-chain pressure on traditional ship formation protection systems are analyzed. Then, key technologies are reviewed from four aspects: reconnaissance and early warning, jamming and deception, strike and neutralization, and terminal damage protection, including multi-domain cooperative sensing, communication and navigation suppression, electromagnetic spectrum confrontation, multimodal deception, layered strike, strike-effect assessment, and platform mission sustainment. Finally, a mission-chain-oriented maritime multi-domain C-UxS protection framework based on reconnaissance, jamming, strike, and protection is proposed, and future trends are discussed in cross-domain integration, soft-hard kill coordination, cost-effectiveness balance, damage assessment, and system-level validation.