Scorn Earth (Su)
At 1st level, an elocater’s feet lift from the ground. From now on, she can float a foot above the ground. Instead of walking she glides along, unconcerned with the hard earth or difficult terrain. While she remains within 1 foot of a flat surface of any solid or liquid, she can take normal actions and make normal attacks, and can move at her normal speed (she can even “run” at four times her normal speed). However, at distances higher than 1 foot above any surface, her speed diminishes to 10 feet per round.
While she remains within 1 foot of a surface, she can make melee and ranged attacks normally, but if she moves any higher, she incurs the penalties on melee and ranged attack rolls as if she were the subject of the psionic levitate power.
It works the same as above water
Ground is not a defined term in the rules, and is commonly taken to mean standing on a solid surface made out of stone or earth. However, the ability also says:
From now on, she can float a foot above the ground. Instead of walking she glides along, unconcerned with the hard earth or difficult terrain. While she remains within 1 foot of a flat surface of any solid or liquid
So the floating "a foot above the ground" here is more specifically defined as floating "1 foot of a flat surface of any solid or liquid". It makes sense to use this specific definition of ground as there is no general definition for it in the first place, and even if there were, this is a more specific one.
The bottom of a body of water is such a flat surface (as would be the surface of the same body). So you can float above either of them as your reference point. The ability says nothing about not working under water, so it should work there, too.
(Even if you went with the traditional take that it must be stone or earth, the floor under water still is made of that, so it still would work).
PS You are not walking or swimming, so it is questionable if you still would be affected by the penalties for underwater combat, which state:
They apply whenever a character is swimming, walking in chestdeep water, or walking along the bottom.
I'd not be surprised if your DM to decides they want to apply them anyways, because logically, these penalties stem from the water's resistance to your freedom to move and swing weapons, not from your mode of travel. But as the rule is written, they would not.