Hey there. Matheson Hammock is my home "port," and I launch there routinely. Additionally, I leave my unattended car & trailer in the marina parking lot for up to a week at a time when I cruise to the bahamas/keys/tortugas etc. I've never had a problem with vandalism or theft in the lot. I do secure the vehicles, don't leave anything in the car, and lock the trailer to the car and attach a "boot" to the trailer. The marina entrance is controlled by security at dusk as well.
If you do go there, and it's on a weekend, definitely try to launch B4 845-9am, and return either early (2-3pm) or later in the day (6-8pm) to avoid the weekend clueless boat-newbies
Good luck!
John