As someone that easily gets bit up by sand fleas...I highly recommend 'Skintastic'.
I've tried many, many solutions & have always gone back to 'Skintastic' or just stayed out of the sand after dusk.
If you are bothered by mosquitoes...I always use something with a higher amount of deet in it than 'Skintastic'...so I don't have to keep reapplying the products that contain less.