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I'm talking about this sort of liquid sand: https://www.youtube.com/watch?v=My4RA5I0FKs
"Desert-as-sea-analog" is present in many media, but often they lean heavily into fantasy, and it seems not many of them have explained how it works. I'm guessing it's a regular desert with some sort of geyser-like system below it that produces steam, but instead of shooting all at once, it creates steam at regular pace over wide area, which produces liquid sand effect as seen in the video above.
In reality? No. But believably? I believe so!
You can't use fluid, this would coat and weight the sand and you'd quickly have mud, which is almost as solid as the dry sand in the first place. It must be a dry gas, fairly evenly distributed along the "sea floor."
As Gimelist mentions, volcanic gasses are the most likely candidate because the volcanic heat is melting things (dry things) and a component of that process is gas. Offhand, I can't think of any other way to create gas in sufficient quantities that this would work.
Note, from the point of view of "realism," you have a problem with a reasonably consistent flow of even volcanic gas. Once something's melted, it requires more heat to get more gas from the same cubic meter of magma. In other words, you need to keep melting new material to get more gas. It's very unlikely that the quantities you need can be achieved. But that shouldn't stop us!
Next, we need a distribution system. You'll notice all that lovely copper pipe in the bottom of the hot tub in your video. Geophysics aren't usually so convenient. But, what if our planet once had a massive coral seabed? Some dead coral is porous, so let's have, say, a large, shallow, sea that is chock full of coral! Then something happened (this "something" will be important in a moment) to drain the sea, and the coral died, and the winds came, and then the sand, and the existence of the coral has become lost to time.
So, we have a source of gas, a distribution system (the coral) and the sand on top. Now we need to get the gas from the volcanic source to the coral in a way that distributes the gas reasonably evenly but doesn't melt, crack, or destroy the coral. Let's talk about that "something" we just mentioned.
Geothermal consequences make their way to the surface in many different ways. Usually, they're narrow ways — like volcanoes. We need that gas to cover a huge area with enough pressure to seep through the coral and bubble up through the sand. I give you...
A caldera.
And not just any caldera, but the Yellowstone Caldera. In other words, reasonable proof that a large tract of land could have, not to terribly far beneath the surface, active magma. We don't want a single large release point of gas (volcano), we want a massive area with — if there was water — mud pots and geysers and hot springs and all kinds of ways for the gas and heat to get to the surface.
Now, this isn't to say that the Yellowstone Caldera is one giant mud pot. But it's a proof-of-concept that a large area with lots of cracks and fissures that don't lead to coral-melting heat can exist. Cracks and fissures that can release gas.
So, underground magma is creating gas, gas percolates up through a large, shallow caldera, encounters the porous coral sea, which distributes it along the sandy sea bed, creating the sandy sea you're looking for! What are our limitations?
Dry! You can't have water. That means no aquifer. I'm not convinced that's possible (water has a habit of getting everywhere), but maybe our planet's a bit like Arrakis, where the water has been conveniently contained by the Little Maker vector of the sandworms. Maybe the "ocean" is surrounded by the massive crater of the caldera, guaranteeing no rain falls on the sea, whatever, you can't have fluids anywhere near this sea or it won't work.
Shallow! While the video shows a couple of feet of sand in the hot tub, it isn't showing how much air is being pushed into the system. I suspect it's a horking lot of air. The deeper your ocean, the more gas is required, the higher the pressure under the coral, the more likely you'll break up the coral and ruin the effect. So your boats will all have shallow keels, I suspect the "ocean" would need to be no more than 2-3 meters deep.
Small! You can't have planetary oceans, like the Atlantic or the Pacific here on Earth. That's simply too much area to explain with a caldera (or any other means of generating/distributing the gas). I think you could believably argue something the size of the Mediterranean sea is as big as you could believably make it (without simply ignoring believablity, which is how everyone in the past did it).
HOT! Not hot enough to burn the boats maybe, but definitely hot enough to cook the proverbial egg. Remember, volcanic gasses! No water nearby and you'll be sweating like a stuck pig. Better bring plenty of water with you! This will also affect the design of your boats, which won't want to have sealed areas below decks. Think "oven."
Geologically stable! Which is a somewhat difficult sell considering we're depending on a caldera. But, if you permit volcanic actions or earthquakes, you'll break up the coral and the whole thing goes to pot.
Note that the coral will never distribute the gas evenly. This means you'll have a believable argument for currents, areas where you can move quickly and areas you must slog through, even islands! No trees, but the lizards definitely want to make the acquaintance of new water sources!
Finally, you can have a ton of fun with the flora and fauna of this ocean (which might be used to contribute to the dryness requirement — if they pick up every drop of water fast enough!). I'm thinking it! I know you are too! Land shark!
May the mighty Sarlacc never find your ship!
Perhaps a vast colony of lithotrophic organisms lives deep beneath the sand?
Long ago, the region was a rocky plain. Then, a species of lithotrophs was introduced, and they began to consume the very rock. The resulting by-products? Sand and oxygen rich gas.
Over millions of years, the lithotroph colony has excavated deep into the crust, leaving above it a vast desert. Their gaseous excretions acting to liquify the sand above.
Boil / condensation.
Imagine a world where the atmosphere was mostly medium chain alkanes - like gasoline. At night the atmosphere largely condenses into liquid and trickles down thru the sandy surface to deep pools. In the day, the heat of the sun causes the alkanes to volatilize. They bubble forth from under the sand, causing the effect you want.
This offers a method to regenerate the bubbling sand fields daily, indefinitely.
No Not on an earthlike planet. the nature of sand means once it gets thick it becomes immobile, worse at only a few meters it begins to reconnect into a solid thanks to pressure welding, if you somehow forced it to move it would be so abrasive it would grind itself into silt or clay in weeks.
There are two ways to help, ideally you want to combine them, but even then it is really iffy.
it is not sand but a sand like organism, that opens up some options at least although not great ones. preferably something that grows a near perfect spherical super durable shell.
Use a lower gravity, sand compression will not be as big of an issue, but you will still have the abrasion problem.
