There is theft but no more so than in any other tourist area. All you need to remember is to take ALL of your valuables out of your car. If you put them in your trunk they smash out a window and open it. Likewise at the beach, try not to take anything of value and, and always keep it in sight. It also helps to ask a friendly looking couple to keep an eye on your things if you go and take a dip. Just use common sense and you'll be fine!
Aloha!

We had our convertible top slashed while in Hawaii. We learned that kids target rental cars and figure even if they get caught no one will come back to testify.

After that, we learned from the locals..and always left our car unlocked with windows rolled down at trailheads and beaches. At these locations, there is a good chance you are being watched by someone to see if you are foolish enough to put valuables in the trunk for "safekeeping" or if all your luggage is there. Once you walk away from your car, they have plenty of time to smash and grab.

In addition to the above advice, I know several surfer friends who always take some kind of cheap rear-view mirror adornment (fuzzy dice, fake baby shoes, you name it) and hang it in the rental car. They claim it makes the car look less touristy and thus less of a target. Hey, for the $ you'll spend, why not.