Whatever you hook to the roof can't block the sight lines below it, which means you would likely have to mount it bottom down, pretty close to vertical.

At this point, any conventional boat becomes like an aerodynamic surface and could lift off the car

Which got me to thinking about these boats which I saw advertised in a Boy Scout magazine over 40 years ago. More like carrying a sheet of plywood atop the roof, which might make things more manageable.

Trunk's too small for an inflatable

hope this helps