Items that are not secured to the boat will also move along with the rolling motion. As the boat moves from upside down to slightly to its side, the bag would follow the lowest point of contact in the boat and essentially follow the hull line.
Water is heavy, but I really don't think the bag would be so large that it would hinder rolling. I've seen people re-enter and roll completely flooded hulls and I can't imagine it would be worse than that.
I tend to not like things to be loose in the boat, however, so I try to find solutions which keep items secured as much as possible. One could also find/make a container for your water bag that could then be secured in the boat (either behind the seat or a place where it won't impede comfort or mobility).
Kayak Motor Kit
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