This was dh's first time traveling with a backpack and he LOVED it. He will never go back to his wheeled carry on and separate computer bag EVER again. His Pacsafe backpack had a separate padded compartment solely for the 15" laptop. TSA security has always been difficult for him trying to juggle the laptop and this made it so much easier.

There are two pairs of zippers. The main compartment as well as the smaller front one. If you go to this REI link and mouseover the zipper you will see that there is a hole in each zipper.
We strung the retractable cable through each hole and then around the post of the palapa. The cable is 2.4mm / 0.1in diameter high-tensile stainless steel wire wrapped in this plastic to make the surface smooth.
No way anyone is going to get through that without someone on the beach seeing it.
It was easy to hop in the water to cool off or go off to lunch without having to lug a beach bag and other assorted gadgets with us.
Pacsafe meshsafe b200 laptop backpack is discontinued but available online via Google search.
This is the lock we used with the backpack, the Pacsafe retractasafe 250 4 dial retractable cable lock US$ 17.99
Pacsafe Youtube Channel for online demonstrations of their security features