There is one problem you simply can't overcome: the density of sand is greater than the density of water. For something to float, it must have a density less than water. Because sand's density is greater than water, it will always sink.
Condition #1: A lot of sand, some water.
Yours is a desert world, but even desert worlds have bedrock. So let's assume an area of bedrock shaped like a bowl but filled with sand. As you add water, the sand gets soggy. If you don't fill it too fast, you eventually get a bowl full of wet sand. You can have life in this condition, but not fish. "Swimming" (as we traditionally think of it) isn't possible.
Condition #2: A lot of water, some sand.
Consider the same bowl full of sand, but this time we'll add water fast enough that erosion can occur. Eventually you have a bowl full of water with some sand on the bottom. Even if we consider this like a sea (no outlet), you'll have salty water, but it's still water with a sandy sea bottom.
Condition #3: We take advantage of surface tension
A finely ground sand (a small grain) can float on water! It's not that the density is lower than water, it's that the mass of the grain is insufficient to break the surface tension of water. You can make the surface absolutely opaque in this way. However, as the amount of fine grain sand builds on the surface, you'll occasionally get globs of sand that fall beneath the surface and sink. How thick can the sand get? Not very. While the thickness varies greatly with the nature of the sand and the salinity of the water, etc., I'd be surprised if you could get a consistent thickness much more than a millimeter or two.
Condition #4: Sand... made of igneous rock.
We can circumvent some of the problems of sand density using igneous rock. What is igneous rock?
Igneous rocks (from the Latin word for fire) form when hot, molten rock crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface. Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies. (Source)
The beautiful thing about igneous rock is that it's common for air to be trapped inside the rock's porous interior. The rock has a higher density than water, but the aggregate density of the rock (due to the buoyancy of air) is less than water! Now we're cooking with gas! You can get a LOT of rocks floating in water this way.
For a while...
The other problem with igneous rock is that they're, well, porous. Which means the water will eventually seep into the rock, displace the air, and the sand will sink.
Condition #5: Pumice Stone
But there is one option: pumice stone. Pumice stone is a specific type of igneous rock. It can float for a long time... but not forever.
When Havre erupted in 2012, it produced a large amount of floating pumice so we know the eruption was explosive. Ocean currents swept the floating pumice along, some of it as far as the eastern coast of Australia. Pumice doesn’t float forever—water seeps slowly into the holes in the pumice, making it heavier and heavier until it sinks to the seafloor. (Source)
There are temporary ways in Real Life to get what you want: a sea of soggy sand that's mostly water. But Real Life is against you and no permanent solution exists. You can get closer using biomatter (swamps, peat bogs, etc.), but even that might not be quite what you're looking for. There is, of course, nothing stopping you from declaring that your world has exactly the kind of sea you're looking for and ignore the explanation. But if you need the explanation, I believe temporary is the best you can get.