If someone were to cast Reverse Gravity and included an area with deep water in it (such as a lake, ocean or deep riverbed), what would happen?
Would the creatures be pulled out? Would the water move to the top?
The spell only affects creatures and objects:
All creatures and objects that aren't somehow anchored to the ground in the area fall upward and reach the top of the area when you cast this spell. A creature can make a Dexterity saving throw to grab onto a fixed object it can reach, thus avoiding the fall.
Water is a feature of the environment and therefore not an object as this line from the Player's Handbook implies:
You can also interact with one object or feature of the environment for free, during either your move or your action.
Also, an object is...
[A]n object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.
... according to the Basic Rules so a liquid cannot be an object as it is not discrete. Discrete objects can be easily distinguished and separated from other objects. You can't separate melted ice cubes (which themselves are objects) from the water that is left behind. (Thanks @Rubiksmoose)
All in all, the water is not affected.
From my point of view, the area itself doesn't have reversed gravity entirely (as that would cause much weirder effects) but instead the current force of gravity acting on all objects and creatures in the area is inverted.
The spell says:
All creatures and objects that aren't somehow anchored to the ground in the area fall upward and reach the top of the area when you cast this spell.
so all creatures and objects do, indeed, "fall" upward.
We could argue about whether water counts as an "object", but we actually don't need to, because the spell also says:
This spell reverses gravity in a 50-foot-radius, 100-foot high cylinder centered on a point within range.
and we know that water is affected by gravity.
Simulating the exact mechanics of this would be complicated, but we can expect:
This continues until the spell's duration runs out.
A very detail-oriented DM might invent rules describing how fast the creatures at the top were pushed to the edges by the water current, and what athletics checks they might need each round to avoid being swept off the spell and falling back to the lake surface (and then being swept back upward again, et cetera).
A less detail-oriented DM might just tell everyone they were at the top of the spell and there was also a lot of water up there.