If you are really serious about kayak diving but have transportation or storage issues, you should look into folders or inflatables. I have a 17 foot Klepper Aerius II and a 17.5 foot Folbot Super...both would fit inside your Fiat quite nicely. With the sunroof closed. The Klepper is a darned sight easier to assemble and is quite a bit better boat...and is much more expensive. But they are both great boats and will hold a tremendous amount of gear...I have the sailing rig for the Klepper and have gone on some crazy trips with it...you can even sleep in it in a pinch if you are travelling solo.
My real recommendation, though, is to check out Innova's line of inflatables. I have a Safari and a Twist. I cannot recommend them highly enough. The Safari is self bailing with a removeable skeg. It tracks unbelievably straight for a 10 foot boat. It is made of hypalon and is incredibly durable and well constructed. Deflated, it will fit into the overhead on your next flight. It weighs 24 pounds. Like I said, an incredible boat..it is even a great surf kayak.
The Twist is even lighter and packs even smaller. It weighs 16 pounds and packs down so small that sometimes I bicycle down to the boat landing with the kayak in one of my panniers. The Twist does not track as well as the Safari, though, and I don't believe the "litepack" material it is constructed from is as durable as hypalon, but for what it is...it is a great boat. Both are stable enough to be used as Yacht tenders in a pinch.
The Safari is the choice for a solo diving inflatable Yak, though. Wet reentry is a snap and there is plenty of room for gear. It is great for exploring hidden areas.
Remember Frank Zappa's song "Don't eat the yellow snow", well don't drink the yellow water unless you have treated it and boiled it first.
