Okay, what we would call mortar sand around here. Yep, it's much finer than concrete or play sand. I'm thinking it will be tough to hold it in place, without it washing out into deeper water. It's fine enough to move when the wind blows over top, let alone the rain. I don't think it will hurt a thing, just not sure how long it will last.
+1 If you get the sand free, go for it. If you have to pay for it, I would use pea gravel or some slightly larger rounded stone. It will stay in place longer and still not be too bad to walk on. IMO a mesh type underlayment will keep whatever you put down in place longer.