OK you motivated me to look up the biological half life of tritiated water which is about the same as regular water, it doesn't isotopically partition very much in the body.

http://en.wikipedia.org/wiki/Tritiated_water

There's an interesting wikipedia article about total body water where you basically figure out how much water is in your body in an absolute sense by consuming some D2O (or I suppose for those who want to live on the wild side, T2O) and the results keep coming up as a half life around ten days in the body. It just doesn't bioaccumulate. I think you're thinking of the heavy metal boneseekers or radioiodine accumulating in the ... gland thats by your throat (I forget?)

You are correct that mathematically you never really excrete all the T2O. However once the level decays below a banana equivalent dose, it just doesn't matter anymore.

http://en.wikipedia.org/wiki/Banana_equivalent_dose