Should be made of tight weave material to keep out sand.
Lightweight so it can fold or roll in your luggage. No heavy leather base for instance.
Opens wide and stands up with lots of room for getting out drinks, big hardcover books, towels. Parachute nylon would be worth considering for its stiffness.
Zippered top so you can close it completely to the beach environment.
Zippered inner pockets for extra sand repelling for cellphones or sandwiches.
Shoulder strap and handle in dual configuration for flexibility.
Rivets, zippers, and grommets of stainless steel to resist wear and beach corrosion.
Washable. Colorfast. Maybe prewashed.
Bright spaceage color and distinctive pattern so you can easily find it if you've coming out of the water, or spot it from the water to check your position and drift. Pink elephants dancing on a bright green lawn. Square dancing. It's a hoedown.