The ships have boilers (most oil fired, but some also have exhaust gas waste heat boilers) that generate steam. This steam is used to heat domestic hot water, and laundry water, but this is just really a secondary use. The major use for the steam is to heat the fuel for the engines.

Water on a modern cruise ship is primarily heated through an ingenious process called waste heat recovery, which makes the ship significantly more energy-efficient. As the massive diesel-electric engines run to provide propulsion and electricity, they generate an enormous amount of heat. This heat is captured from the engine's exhaust gases and cooling water systems using heat exchangers, which then transfer that energy to the ship's freshwater supply for use in showers, galleys, laundries, and swimming pools. When the ship is in port or the engines are running at low capacity, supplemental heat is provided by oil-fired or electric boilers. Increasingly, newer "green" ships in 2026 are utilizing high-efficiency heat pumps and thermal storage tanks that function like giant thermoses to keep water hot for later use. This integrated system ensures that the thousands of gallons of hot water required daily by guests and crew are produced with as little additional fuel consumption as possible, recycling energy that would otherwise be wasted.