Final answer:
The Pacific Ocean experiences unusual warming during El Niño, a phase of the ENSO cycle affecting global weather patterns and marine life.
Step-by-step explanation:
The body of water that becomes unusually warm due to El Niño is the Pacific Ocean. El Niño refers to the warm phase of the El Niño Southern Oscillation (ENSO) and is characterized by a band of warm ocean water that develops in the central and east-central equatorial Pacific. This event affects weather patterns globally, leading to extreme weather in some regions and influencing ecological systems, such as causing large die-offs of fish and marine mammals due to warmer waters, particularly near areas like the Andean coast.