[svandamme]

unless it's distilled water, and you know the exact temp...
and the precise mass and size of the object and the object is perfectly round...
it will vary to much

And yes, size does matter, the bigger it is, the more drag...And if the size of hte object get's close to the size of the container of water... you'll get a piston like pressure build up that slows it down even more...