The length of your shorts could be knee level or a bit before the knee. If your legs are not in the best shape please wear knee length board shorts. Being able to flaunt you torso on the beach is dependent on whether you're properly waxed and your abs are in the best shape :o) if they're not please wear a vest or t.shirt. Have respect for other people who came to have a good time :oD. Clothes are meant to flatter our assets not flaunt our flaws.
Check out the wonderful selection of shorts at Asos if you're looking to buy.