Decide what you can leave behind and what you must have with you; leave everything possible locked in the trunk of the car. The less you actually have with you, the less that can be stolen. If you must take pictures with a non-waterproof camera, take your pictures and then lock the camera in the trunk before you get into the water.
Store small, necessary items like your car key, spare cash and credit cards inside an Aquapac or similar small waterproof pouch on a lanyard. Make sure your waterproof pouch is secured to your body as closely as possible, and don't keep anything that can be damaged by salt water inside. If you must keep your cell phone on you, put it inside a waterproof baggie with a zip seal before putting it in your waterproof pouch for added protection.
Use strategies to make any valuables you still have out at the beach hard to steal or unattractive to thieves. The key to this is in the type of bag or container you use.
Start with your bag's appearance. Bright colors draw attention not just from thieves, but from everyone at the beach, and are less likely to be disturbed. Similarly, bags with childish prints or cartoon characters are often overlooked because it's assumed their contents are not valuable. Old, beat-up bags draw less unwanted attention than designer leather.
Certain bags are inherently easier to secure as well. Bags with long straps and lots of pockets are perfect if you have a tree or permanent post to secure them to; long straps can be knotted and tangled to the point that a thief will give up on carrying the bag away, and few will sit in front of all the other beachgoers to rifle through tons of small pockets. You can also use laptop locks to secure your computer and similar valuables to a post or tree.
Choose a spot for your blanket and possessions close to the lifeguard, then strike up a conversation with the lifeguard and/or a family near you. Make sure people remember you, and don't be shy about asking them to keep an eye on your stuff for short periods of time. If you're generous about sharing things like cold drinks, you can generate enough goodwill to ensure your possessions are watched over even when you're not there.
Check on things periodically, and put everything in the car trunk if you feel uncomfortable for any reason. Trust your gut. If a suspicious-looking person is checking things out, or too many kids are running around unsupervised, trust your instincts and lock everything up, even if it's inconvenient.
