Theoretic as that may be, I can move about much easier without water inside than I could when it overflowed the top. In the case that happened to me when I was young and unaware, they filled and toppled me over and I'll tell you, trying to right myself was quite the task. I was very lucky to not drown. A friend of mine who worked with ADF&G did drown one year (1985 or 1986) in the same river, when he fell over and couldn't right himself. Theoretically there isn't any additional "weight" per se, but try it sometime and see just how difficult it is to regain your footing.
