In a different evolutionary timeline, humans evolved, some become an aquatic race, some become a land dweller, and some capable of living both on land and underwater.
Due to the limited amount of land available (maybe a 90% water/10% land ratio), only the most powerful and wealthiest could live on land.
However, someone found a way to harness the power of something to raise and lower the water level of the planet, causing there to be much more land and thus an easier opportunity to live on land. Changing the water level so drastically had catastrophic consequences and mass casualties since at this point the wealthy race adapted to live solely on land, some only in the water, and a vast minority that stayed amphibious. This something is only available every several millennia.
My main question is simply: How a sea-dweller submerge the entire planet and make sure it cannot be undone? Consider this is when the something is available.
I already have a few ideas but I do not want to simply describe a comet or something else that's overdone because I want this to be a little more original.