The sand along US 90 costs nothing to use and nothing to park beside, and both of those are unusual on the Gulf.
What you are getting: an artificial beach, pumped and graded in 1951 and 1952 to protect the seawall behind it, running about twenty-six miles along the Harrison County shore. It fronts the Mississippi Sound rather than the open Gulf, so the water is shallow a long way out, warm from May into October, usually tea-coloured because five river systems drain into it, and effectively without surf.
That makes it excellent for wading and paddling with small children, pleasant for a long flat walk, good for kite flying in the sea breeze, and unsuitable for anybody who came for waves.
The caveats are genuine.
Check the water-contact advisories. The state environmental agency samples the beaches and posts advisories, and after heavy summer rain they are common rather than exceptional. In 2019 prolonged Bonnet Carre Spillway openings put an estimated ten trillion gallons of river water into the Sound over a record 123 days and closed all twenty-one Mississippi beaches to water contact that June.
There are no lifeguards along most of it.
There is almost no shade. The dunes are low, the road is right behind you, and you should bring your own.
Shuffle your feet in warm months for stingrays, and expect jellyfish in late summer.
Sand is not the whole coast. The piers along US 90 are free to walk out on and give you the water without wading into it.