I never had a problem, with either the kayak, or the inflatable. Maybe it was because I had so many tie down lines, one on front, one on back, two lines on each side holding to the roof racks. I have never lost a boat, canoe, kayak, or inflatable. I guess it is too much trouble to get all the lines off, I guess that is why I use to find myself in the surf rather than the kayak, too much trouble getting it off.
A bicycle cable attached from the roof rack, around the seat should take car of thievery in most cases.