Quick Answer
More water would not simply mean larger oceans. Sea level would rise dramatically, continental shelves would disappear beneath the sea, ocean circulation would change, and many present-day coastal and inland ecosystems would be lost. The exact result depends on where the added water came from and how quickly it arrived.
The first thing that changes
If an enormous additional quantity of water appeared on Earth, the most obvious effect would be a rapid rise in sea level. The exact rise cannot be inferred from a simple percentage because the shape of ocean basins matters: water would flood low-lying continental shelves first, while mountain ranges and high plateaus would remain above sea level for much longer.
Coastlines would move inland
Modern coastlines are tied to a particular sea level. Raise that level substantially and cities, ports, river deltas and agricultural plains would be among the first landscapes to disappear beneath the ocean. Some present-day seas would merge into much larger bodies of water, changing the geography of entire continents.
Established scienceThe amount of land flooded would depend on the final sea level, not simply on the amount of added water. Basin geometry and the distribution of water are crucial.
The climate would reorganize
A larger ocean would store more heat and moisture. Evaporation would increase in many regions, while cloud patterns and precipitation would shift. Ocean currents would also be altered by changes in basin geometry and density gradients, making regional climate outcomes difficult to predict without a detailed climate model.
Life would gain some habitats and lose many others
Marine ecosystems could expand into newly submerged continental areas, but terrestrial ecosystems would lose enormous amounts of habitat. Freshwater systems would be especially vulnerable because saltwater would move farther inland in many regions. Human food systems would also be disrupted as productive plains and river deltas flooded.
The long-term picture
Over thousands to millions of years, erosion, sediment transport, ice-sheet changes and chemical cycling would reshape the new coastlines. Earth would still be recognizable, but the distribution of continents, shallow seas and productive ecosystems would be radically different.
The bottom line
More water would make Earth more ocean-dominated, not simply wetter. The biggest changes would come from geography, sea-level rise and altered ocean circulation, with consequences spreading through climate, ecosystems and civilization.
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.