As far as the "fractional tiles" vs. "units of water" debate goes, remember that the unit systems are currently all over the place for liquids and thus it's a bit silly to try to find The Answer. When a dwarf fills a bucket from a pond, it then contains "water [10]" which if poured into a pond results in 1/7 water and if dumped on the ground (i.e. by [f]orbidding the water and [d]umping the bucket, not poured out as a pond-filling job) creates a water 'contaminant' that in adventure mode represents an infinite source.
Keeping in mind, though, that Toady canonized the size of a tile for purposes of calculating minecart physics -- if I remember correctly it is two meters square by three meters tall -- and seven bucketfuls of 'water [10]' fill that volume completely. So it's actually quite simple to develop a consistent unit system for liquids that we can then compare the exceptions to. Intuitively we could call these units "Drink," "Bucket" and "Tile" and say that 10 Drinks = 1 Bucket, and 7 Buckets = 1 Tile. These would fit relatively closely to real world equivalents of 'cup,' 'gallon' and 'barrel,' if that helps you visualize them. Using the Dwarven Standard system, along with the fact that 1 Tile converts to a real-world volume of 12m^3 we could then determine through various comparisons the volumes of many other objects in the game -- for example, a calculation could be performed using the weight and material density of a forgotten beast (currently only possible in corpse form, but still) allowing us to determine its volume easily. Similar comparisons would settle a number of questions in the minds of a number of players as to the absolute and relative sizes of various creatures and objects.