Quick Answer
Living underwater would require more than the ability to breathe water. Humans would need adaptations for pressure, temperature, buoyancy, communication and long-term access to energy and food. If those problems were solved, underwater settlements could expand into environments currently occupied mainly by marine life.
Breathing is only the first problem
Human lungs are designed for air at relatively low pressure. Permanent underwater life would require a completely different respiratory system or technology capable of extracting oxygen from water efficiently.
Pressure becomes important
Water pressure increases rapidly with depth. A human adapted to shallow water could still face serious limits farther down, so underwater civilization would likely occupy a range of depths rather than the entire ocean.
Cities would need energy and materials
Underwater buildings would need protection from storms, currents and corrosion. Energy could come from floating solar systems, offshore wind, tidal power or subsea infrastructure, while food could come from aquaculture and marine agriculture.
Transportation would change
Oceans would stop being empty space between countries and become inhabited territory. Fast underwater transport could connect coastal settlements, while conventional surface shipping would share routes with a much larger permanent population.
SpeculativeA biologically adapted human is hypothetical. The engineering and ecological consequences can be explored, but there is no evidence that modern humans could simply adapt to permanent underwater life.
The ocean would become someone's home
A permanent human presence underwater would create major conservation questions. Waste, construction, fishing and energy production could disturb ecosystems that currently have little direct human infrastructure.
The bottom line
Underwater humans would transform the ocean from a resource and transport environment into inhabited territory. The largest challenges would be physiology, energy, infrastructure and ecological impact.
Permanent underwater living would require solving several independent constraints: oxygen supply, pressure, temperature, waste removal, food production and long-term effects on the body. Breathing underwater is therefore only one part of the problem. A realistic scenario would likely resemble an underwater habitat or specialized life-support system rather than a human body simply adapting to breathe seawater.
The engineering problem
A note on our approach: Every article on WhatIfLab separates what current science establishes from what remains genuinely speculative. Where we cite a figure or finding, it reflects published, mainstream research at the time of writing — not a prediction dressed up as fact.