This one is a little more unorthodox than some of the other answers, consider it a wildcard. I'm aware it's not spherical, but it fulfills all of the other criteria and actually negates the need to be spherical. The OP did say that being spherical was just 'an idea' so I assume (s)he is open to other possibilities.
I propose a plant-animal hybrid.
Meet the Chryvador. A play on the word Voador which is Portuguese for 'flying' and the feeling that any creature that looks like this should absolutely have some form of 'chrys' in its name.
A large but lightweight creature, not unlike the Extatostoma tiaratum (pictured)
While the actual creature in the image is in fact an insect which pretends to be plant to hide from predators, the Chryvador would have far larger 'leaves' with the ability to act as sails, so it is carried by the wind but is able to control its direction in a similar manner to that of a sailing boat.
The Chryvador does not need to store a lot of water, only around a day's supply at most. It has a sharp and extremely long extendable tongue which can penetrate the ground rock and down into the water table several meters below the surface and extract the water it needs to survive in a similar way to a tree root. If approached by a predator while 'drinking' it can save time by detaching its tongue and taking off immediately; the tongue will grow back in a matter of hours so while it is possible, it uses a lot of energy and is not desirable to do this regularly.
Generally living in groups of between twelve to fifteen, they will display behavior not dissimilar to that of the Meerkat. To extract enough water from the ground below the rock a large amount of time will need to be spent doing this. Two or three will keep lookout while the rest perform this act. Every individual in the group will take its turn as lookout.
As an additional defense mechanism, when a tongue is discarded it will continue to extract water on its own and at the top a plant will grow which is smaller and inert but looks very similar in appearance to a Chryvador. These are left in place for two reasons. Firstly they are useful decoys. But secondly they provide a source of nourishment for other nomadic creatures. It is often in the interest of these other creatures to scare off the Chryvador but not actually kill it, as this provides them with a regular source of food and water for themselves. These creatures will actually defend the Chryvador from animals seeking to eat it, as that would essentially cut off its own food supply.