Tenser's floating disk cannot move across liquid.

Merriam Webster defines ground as

the bottom of a body of water

or

the surface of a planet (such as the earth or Mars)

So I don't think "3 feet above the ground" can mean "3 feet above water" by any stretch of the word. However, the first definition indicates that the disk may float "3 feet above the bottom of a body of water".

Moreover, Merriam Webster defines terrain as

(1) : a geographic area (2) : a piece of land : GROUND

or

the physical features of a tract of land

Given the context, it is most likely that terrain is being used as a synonym for ground.

And finally, I don't believe a "body of water" quite fits among "stairs, slopes and the like".

My interpretation.

Nowhere in its description does Tenser's floating disk mention liquids, or anything related to liquids. The spell only describes features attributable to solid surfaces when it explains the disk's capabilities:

It can move across uneven terrain, up or down stairs, slopes and the like, but it can't cross an elevation change of 10 feet or more.

As such I do not think the disk is meant to cross liquids. Anything between liquids and solids is undefined and up for DM adjudication. If you ask me whether the disk can float across a particular surface, I would ask: "If this surface were a steep slope, would it maintain its shape?"

Water? Nope.
Ice? Sure.
Snow? If it's packed sure, but the disk may plow through the fluffy snow.
Copious amounts of jelly? Probably not, but I'm unfamiliar with copious amounts of jelly.