The king said: “Venerable Nāgasena, you say this great earth is established on water, water is established on air, and air is established on space. I do not believe this assertion.”
The elder took some water in a water vessel and explained to King Milinda, “Just as, your majesty, this water is supported by the air, so too is that water supported by air.”
Here is a modern-day video demonstration that I found:
I think it might be better to use current astrophysical knowledge to try to make sense of that.
Water here could refer to the liquid mantle.
Air might refer to the atmosphere on the opposite side of the earth?
Space … earth is in space.
Anyway, the trick is, gravity doesn’t point in only one direction when you zoom out to space. It becomes pointing to the centre of mass of the earth.
I admit this mapping is unsatisfactory, especially on the air part. Well, there’s pockets of natural gas in the earth. But usually that’s above the liquid mantle.
Space at least is the easiest to make sense of.
Indeed, the video you posted likely is what’s referred to in the text. Air pressure supporting water.
For, without atmosphere, our ocean would just evaporate to space. So in a sense, if we don’t view that support means downwards, but enables the existence of, then it makes total sense.
Ocean can sort of serve as a fast cooling mechanism for ocean volcanos making new lands, the ocean is prevented from evaporating into space by the atmosphere, and the atmosphere is trapped by gravity, and protected by magnetic field from being blasted by solar wind into outer space.