I grew up in a SoCal beach town and still live in a beach town. If passers-by can easily know your car is a rental ( window stickers, license plate frame etc, ) then a dry bag is the way to go, if you like to skin dive, the buoyancy of the dry bags can be problematic.
Since you’ll be leaving your luggage in the trunk in any case, park it in an area with high-traffic. Thieves get nervous when there are lots of eyes around that potentially can see them. You could place your money belt in the trunk underneath the mat in the space where the spare tire is kept. Be sure to hide it out of view deep down in the wheel well. Do this before you arrive at the parking area so no one sees you hiding things in the trunk. Another precaution is to wear street clothes to the beach with your swimsuit underneath, or put your beach gear and towel in your shoulder bag and change at the beach.
Again, do this before arriving where you park the car.
Savvy thieves simply watch cars being parked and if you walk away from the car in a swimsuit, well, they know you’ll be gone long enough for them to ransack the car.
That still leaves you with the car key fob which you’ll need to put it in a waterproof container and secure it in a zippered or Velcro pocket in your swimsuit.