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.