If you're visiting Maui, I highly recommend the drive to Hana. Hana is actually pretty boring, but everything on the way is what the drive is about. The end for most is the seven sacred pools, but I would continue on and drive the back side of Haleakala. It turns from a lush rain forest type environment to very dry and finally you end with a drive through the lava fields. It's pretty amazing and you'll see views that can't be matched anywhere. Pick up the book Maui Revealed and it will provide directions to every waterfall, hidden beach, freshwater cave, lava tube, etc. You can plan your trip very well and hit the spots most tourists miss.
You're going to want to get out early (be going through Kahului by 8am) to beat the majority of tourists to the popular spots, i.e Black Sand Beach and Seven Pools.
In regards to how fast people drive on the road ranges from a turtles crawl by most tourists and what seems to be recklessly fast by the locals that drive the road every day. If you see a local coming up behind you, pull over to the side and let them by.
As for the diving I can't comment. My first dives in Maui will be this February, can't wait!