One more recommending you keep the boat rolled and in the trunk. I've seen folks driving around with inflated boats on the roof, but only when they were shuttling back to the put in for another lap, not for the whole summer. The high altitude UV won't be good for the material and the wind drag may just pull that rack right off the car once you get out on the highway. An inflated boat going over the passes will also puff up quite a bit and then get squishy at the bottom.
With a good 12 V inflator and a small top-off pump you should be able to air up that little boat in no time anyway. Also, get a tarp to lay the boat out on for inflation and deflation (so you don't roll it up with sand and grit in it) and use it to wrap up the PVC boat with when it's rolled to prevent abrasion damage.
Have fun - I bet this little raft will be a blast!
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse