Even if you fill the canoe with a second smaller copy of it, won't that copy still be a 3D-surface, like other Carrara objects? In other words, it only has polygons on its surface, not in its interior. But I admit, if we were trying to animate a canoe moving along, it would be a bummer having to animate a boolean all the way. We would probably have to use a low camera angle, so the viewers couldn't see inside the canoe.