If I have a situation where an ooze, such as a Gelatinous Cube, is near a source of water, like a small pond or underground stream, is there source material somewhere that says whether the ooze floats or sinks? Is there any mention of their density — so whether they could travel over the body of water, possibly try to resist being carried away by it, or if they sink to the bottom?
Digging around, I can't find a definitive Monster Manual based answer here. But, there is a little piece from one of the published adventures for D&D 5E.
From Out of the Abyss (Spoilers, obviously)
Taking place in The Oozing Temple encounter, which is steadily flooding with water,
we get this little snippet:
Development: If Glabbagool is with the party, the intelligent gelatinous cube floats upward as the water rises and squeezes through a crack in the ceiling to escape the flooded temple and remain with the characters.
The only other possible answer I can give you is rooted in Science, which has a shaky relationship with a Fantasy world anyway.
In order to float, you must have a lower density than the stuff you are trying to float in. Average density is mass divided by volume. Of all things in D&D, a Gelatinous Cube is literally the easiest possible thing to compute the density of.
We know that an average gelatinous cube is 10 feet on a side...1,000 cubic feet. And, according to the 3.5E SRD, a Gelatinous Cube weighs 15,000 lbs. Divide that out, and that gives us a density of 15 pounds per cubic foot.
Convert to metric for comparison purposes and we get 240.277 kilograms per cubic meter.
Fresh Water has a density of 1,000 kilograms per cubic meter. (Salt Water is even more dense)
Thus, a Gelatinous Cube has a lower density than water. According to RL Science, it should float, and float pretty high in the water, too.
Dragon Magazine #124 states that gelatinous cubes can move around in water "freely", but their acid is diluted. This makes me think that they are allowing some of the water to flow through them... but either way, free movement in water I think means it can go in any direction it wants. Scientifically/logically, I'd wager that it absorbs water from the front and expels it from the back to get propulsion. And it absorbs as much or as little water as to make it closer to the density of water to help it go up or down... but then uses the propulsion method for the majority of its movement. So it might not have to float, as it has the ability to tread water or swim beneath it would how I would play it.
As an added piece, you might say if it has too much metal floating around in it, it struggles to swim/float upwards and would have to expel it. Being mindless, the DM would have to make the call on when it would expel the metal.