Yes it is possible for maggots to enter human skin.
Maggots entering human skin is very rare but it can and sometimes does happen.
The most common type of maggot that enters human skin is the maggot which is fly larvae known as the screwworm.
The screwworm can penetrate a humans skin and go inside of them through the foot, leg or any other part of the body and then live inside of you.
Other parasites from the ground can also live in the human skin and get into the human through there skin as well.
So while going barefoot is nice it's best to have at least a pair of sandals on when walking on the beach.