One thing that has worked out great for my family -- one hour before sunset we put on socks (can even wear with sandals), looks kind of silly but those sand fleas won't bite you, I buy those socks that look like ladies nylons and where them with sandals, I used to put tights on my kids when they were smaller. Works every time, those sand fleas can't jump up over knee high socks. I also always usually wear tights to a stage show with a long skirt if the show is being held on the beach.
And when you are staying longer than two plus weeks, they seem to stop biting.