Hi rkru,
Your question is easy enough to answer:
Diving out of your rental car is no problem on Curacao - just be aware that a few locals might be interested in what you leave behind in your car... So take some simple precautions:
1. Ask your car rental company if it is ok to leave the car unlocked. If so, even leave the windows open while you go diving. Like that anybody who wants to have a look inside your car doesn't need to break anything to do so.
2. Don't leave any valuables inside the car when you leave it, valuables including
a) cash money - use a small waterproof container you can take with you on your dive to take money and car keys with you
b) passports, driver's licence, and credit cards (leave them in the safe at your hotel or apartment)
c) iPods or cellphones (see b)
d) take only underwater cameras with you when you plan to go diving, so they won't be left behind in the car
3. Don't rely on trying to hide anything in the trunk - potential thieves might already be observing you, so they will notice.
Believe me, this sounds more scary than it is in reality; evil is not hiding behind every corner on Curacao, but some precautions help avoid disappointment.
Enjoy your trip, and don't let anybody catch your bubbles
Greetings
Andreas