Most people who have roof racks don't use them for anything other than ruining their aerodynamics, but there are some useful things you can put up there. For example, you can carry a boat around, or you can carry a box full of cargo.
Now, cleverly, that cargo box can double as a boat. Here's something to ruin your car's aerodynamics and aesthetics that's actually pretty cool. Sadly, it's not as cheap as it looks at $1,590.