Get a swim dress with built-in shorts.

Yes, that’s the single most important feature. A swim dress that’s just a skirt will float up every time you go underwater or exit the pool—which is the opposite of modest. You want one with attached shorts (ideally with a gripper or elastic leg bands) so everything stays put.

Fabric matters too. Look for a heavier, opaque material—tighter weave, not that thin spandex that goes see-through when wet. Darker colors or patterns help. If you’re going for full torso coverage, a rash guard top paired with a separate swim skirt or swim shorts is more reliable than a one-piece dress, because you can adjust each piece.

Avoid anything with a flouncy skirt that isn’t anchored. It’s a constant fight. And skip ones with built-in bras unless you’re small-chested—they never fit right.

Try on with a wet swimsuit before you commit. The store mirror lies when everything’s dry.