I don't think they're water, for a variety of reasons. Consider: their color, their amazing uniformity of size, shape, and the "peach line" going down the side.
Water -- or rocks, for that matter -- would IMO come in a variety of shapes and sizes, with a lot of randomness. The unformity speaks against randomness IMO, as does the ubiquity. They're found tossed around the ground in great number, they're found beneath the surface, and, they're found inside of solid rock.
My money's on them being biological, either something like a fungus (there are some very hard fungi here on Earth, which are quite miserly with water), or some plant something like an "air cactus" (not that any such thing exists on this planet). They also look like goat and deer droppings. More like goat, actually, IMO. I've watched my goats toss out a load, and they tend to scatter and roll. Deer droppings, when I see them in the woods or my garden (grrr....) do have the same appearance (spherical, same approx. size as the marsberries), but Bambi tends to place them into piles rather than scatter them like marbles.
As I said earlier, I'm not suggesting they are ruminant turds. Apart from the obvious (ruminants on mars? yeah right), there's that obstinate "peach line" that defies explanation. (And while I've never gotten up close with my goat's contributions, I doubt they'd have that line, even though they do seem to be nicely spherical, with consistent size.)
I really wish they'd show us some recent color images. They've so far sliced several of the berries, but we've no idea what color they are inside. They look black, but, these are black and white images.
The pale blue exterior, combined with a hard, solid black interior, would be consistent with a fungus, maybe something like a schleroderma (which I've probably just mangled).
OTOH, the uniform size is intriguing, especially when combined with the ubiquity. What I'm getting at is that if they were fruiting bodies, it might make sense that they'd all be the same size, assuming that we're looking at a "crop", all at the same maturity. Even so, I'd expect more sample to sample size variation, but, there are some plants that do manage to have fairly uniform size, so let's accept for discussion that they're all at the same maturity.
But, that theory falls apart when we take into account their distribution. Some are scattered on the surface, some are embedded in rock, some are buried in the sand, but all are the same size.
So, what does that leave us? If they don't "grow" (in size), but are "produced" at their final size, that brings us back to the goatshit. Or, perhaps some kind of seed, or "megaspore"? In either case, there'd be an animal or vegetable source that would form them internally, and then eject them. There are plenty of plants that eject their seeds, and lots of fungi do it too (puffballs, etc.)
But in either case, where are the sources?
I see two possibilities (for this theory). One would be a subsurface plant or animal that ejects its waste (or seeds/spores) at the surface, and the other would be a surface plant/animal that either travels, or, grows, produces its seed, then withers and dies (possible eons ago).
Well, there's some more pointless musing. FWIW etc.