The marine "electricity‒hydrogen‒ammonia‒methanol" integrated energy system converts intermittent offshore green electricity into storable, transportable, and tradable hydrogen-based energy products, providing support for shipping fuel substitution, green chemical feedstock supply, and deep decarbonization of industry; it is also an important pathway for promoting the large-scale development of deep-sea renewable energy, enhancing energy security and resilience, and supporting the low-carbon transition of hard-to-abate sectors. This study constructs a "strategic value‒system architecture‒key technology‒spatial layout" full-chain analytical framework, and demonstrates the strategic layout, key technologies, and engineering pathways of the marine "electricity‒hydrogen‒ammonia‒methanol" integrated energy system. It also clarifies the conceptual boundaries between this system and related concepts such as marine integrated energy islands and land-based hydrogen‒ammonia‒methanol integration projects, and evaluates its strategic value from the dimensions of energy security, carbon reduction, industrial synergy, and international competition. Moreover, the study reveals the architectural characteristics of the system. Specifically, the system is based on offshore renewable energy, centered on hydrogen production by water electrolysis, extended through green ammonia and green methanol, and oriented toward diversified end uses. Based on an equal-calorific-value substitution logic, this study analyzes the carbon reduction potentials of hydrogen, ammonia, and methanol fuel pathways, and clarifies the impact of changes in water electrolysis efficiency on mitigation effects. Additionally, it establishes a key technology system that includes marine renewable energy development, offshore hydrogen production, ammonia and methanol synthesis, storage and transportation safety, intelligent operation and maintenance, and carbon accounting and certification. A technology map covering the main chain and supporting technologies is constructed, and priority research directions are identified. Furthermore, the study proposes a phased development pathway of "nearshore demonstration‒regional hub cultivation‒deep-sea scale-up" and differentiated spatial layouts for the Bohai Sea, Yellow Sea, East China Sea, and South China Sea. The findings of the study can be extended to the forward-looking planning of the marine green fuel industry, high-quality development of marine energy, and promotion of major engineering construction.