Mississippi runs a published water-quality monitoring programme for its mainland beaches and posts contact advisories against individual named stretches. Advisories go up and come down through the season, they are specific to a segment rather than to the whole coast, and they are the single thing worth looking up before a swimming day.
The pattern to know is the rainfall one. Heavy rain over the catchment washes runoff into the Sound faster than a shallow lagoon disperses it, bacterial counts rise, and an advisory follows a day or two later. Give it forty-eight hours after a serious downpour and check the list rather than guessing from the look of the water, which tells you nothing useful either way on this coast.
Separately, from June to November this is a hurricane coastline, and the beach is the first thing closed and the last thing reopened around a system. Debris washes onto the sand after even a moderate blow and the county clears it in stages. Local reporting after a storm is more current than any app.
None of this is a reason not to swim here. It is the reason to spend two minutes checking rather than walking straight in, and the same two minutes tell you whether the jellyfish are about.