A note on the crime thing: Curacao has, I feel, been a little maligned in this respect. If you park a car full of valuable things in a remote area then you can expect to be robbed. This is not especially particular to Curacao, I think you'll find it in pretty much any place in the world. Best advice is to not leave anything worth stealing in your car, and most of the dive shops leave their vehicles empty and unlocked. If somebody's going to break in they will do it anyway, and leaving your vehicle unlocked at least saves the cost of a broken window.
I've got to agree with Crowley on this one. Before Judy and I went down there, I had read a lot about the petty crime, on this board and on others, and it had me a bit worried, but once I got down there, and traveled around some of the island I came to realize that this is more hype than the norm.
Petty property crimes are crimes of opportunity, and are not incidents isolated to one area in the world. Granted some may say that it happens less in my backyard, but that can be debated with examples and incidents to support both sides of the argument. The easiest way to avoid these crimes of opportunity is to remove the opportunity. No opportunity, no petty crime.
As Suzy and Crowley have both suggested; if you're going to hit the more remote beaches (the non developed or non staffed ones), take with you only that which you need for the dives, and leave the rest at your hotel. Leave nothing of value in your car while it's parked at the remote sites, and leave your doors unlocked.
Take along a small dry box and put a few gilders, your ID, and your car key in it, and clip it off , or better stick it in a pocket (I hate danglies

) take it down with you.
If you're the obsessive kind who absolutely cannot deal with this little stress, stick to the developed beaches, many have parking lot security, rental lockers, etc. (Cas Abo, Porto Marie, Hook's Hut, Playa Lagun for example.) the diving at these locations is great, and you get the added bonus of having a dive shop and snack bar on site.
HINT: If you're going to hit the developed beaches (a MUST (IMHO) if you have non divers trekking with you for the day) plan your trip so you visit these beaches during the week day as they can pack up on the weekend. Being on those beaches during the week gives you that "alone paradise" feeling that can only truly be achieved when the beach is practically empty; and for that, the weekdays are best.
Renting tanks is easily done on the island as there are many good shops and we didn't have any issues with regards to the quality of the gas we got. We dove primarily 32% while we were there, and the mixes were pretty good for your typical partial pressure fills. If you are staying at a resort with a dive shop, you can rent your tanks for the day from them, and haul them around, (or hit up Suzy and Mark for cylinders) the daily rental cost per tank doesn't vary all that much throughout the island (at least not more than a few gilders on either side) but some shops offer a better package deals than others. (Again Suzy and Mark for example have a pretty good deal on their packages)
OR
If you are hitting the developed beaches, and you are four divers in one smaller vehicle, you can rent your tanks from the beach's dive shop. The more popular developed beaches, Hook's Hut, Cas Abo, Playa Lagun, Porto Marie, Playa Kalki have dive shops on or nearby the beach, as well as snack shops/bars, and many have rental lockers for added security. Judy I and did this and we never had a problem getting air with the only issue being one beach was out of nitrox, so we dove air for those dives.
HINT: If you are going to be diving nitorx and htting a local baech's dive shop, call a day or so in advance and they'll reserve your cylinders for you.
You need not be a Mensa candidate to navigate the island. Get a good map, The Drive and Dive Map that Suzy mentioned is a very good one (the B&B Curacao is better but out of print and damn hard to find nowadays) and use the driver plus navigator method. Judy, who sometimes can be directionally challenged, was easily able to call out the "rights and lefts", and many of the dive sites were well marked on the roadside. While road signage exists, I would suggest that you study the map (it will only take a few minutes before starting out) plan your route out in advance and have you passenger look for signs and obvious waypoints like tourist signage, and village names etc; street signs do exists, more or less, but can a bit hard to see and interpret on the fly.
Remember you are on an island (I know; Duh), but often travelers forget that there is "big city go, go, go, time and "Island time" Don't expect the traffic (yes there is some traffic on the roads during your typical peak period) the roads to be ripping along like a typical highway in North America and Europe. Expect to get stuck behind someone poking along and although the island's only 40 km (30 miles) long, do not time your trip the same way you would at home. I made this mistake the first time Judy and I drove to west Punt (we traversed the island many times during our two week stay) and damn near missed our charter. The roads are good. Not fantastic, not great, but good. The islandÃÔ main traversing roads are two lanes, often narrow, and unlined, with soft shoulders. In other words watch how and when you pass. Watch out for goats, chickens, and the occasional dog; they tend to wander around a bit.
Take your time; and I mean that in all you do while you're down there. It's a beautiful island, with friendly people, great diving but if you head down there will the "go, go, go" attitude you may "See" the island, but miss its beauty.
Cheers!
Rick