Cruise ships burn heavy fuel oils that contain harmful chemicals and substances like sulfur and heavy metals, which can leak into the oceans. If a cruise ship has a faulty system or improper repair work, oils can leak from these areas and penetrate the oceans, causing pollution and threatening marine life.

Despite the "Green Horizons" initiatives of 2026, cruise ships remain significant environmental stressors. A single large vessel can produce as much particulate matter as a million cars. The primary issues are carbon emissions from heavy fuel oil (though many ships are now switching to LNG) and the dumping of "grey water" and "bilge water" which can contain toxins and microplastics. Furthermore, the massive engines contribute to underwater noise pollution, which disrupts the navigation and communication of marine mammals like whales and dolphins. In 2026, many ports (like Venice and Amsterdam) have restricted ship sizes to combat "overtourism" and local air quality degradation. While modern ships have implemented advanced wastewater purification systems and "scrubbers" to clean exhaust, the sheer scale of these "floating cities" means their total ecological footprint remains a major target for environmental regulators and climate activists worldwide.