The cover up that looks perfect folded on a shop rail is often the one that clings to wet skin and goes see through the second you stand up from a sun lounger, and no amount of handling it in the shop tells you that. It's only after a week of actually wearing one over a damp costume, walking back from the sea, that you find out which fabric was lying to you.
Cotton gauze and crinkled cotton are the ones that behave themselves wet or dry, because the weave already has texture built in, so damp patches don't turn the whole thing transparent, they just look a bit more crumpled, which on holiday reads as deliberate. Viscose drapes beautifully in the shop and then clings in a way that's unflattering the moment you've got suncream and sea water on your skin underneath it, and it takes an age to dry, so you end up wearing it damp for hours. Linen is lovely and cool but creases if you so much as sit down, and some people love that look, I don't particularly. Check the stitching under the arms and along the hem before you buy, because that's where cheap versions fray first, and if there are buttons, give them a tug. Wooden or coconut shell buttons hold up fine, but the painted plastic ones chip within a couple of wears and then don't match anything. Ties at the waist or neck are worth checking too, a tie that's sewn in badly will work loose by the second wearing and you'll be retying it constantly, which gets old fast.
Where people go wrong is pairing a cover up with a costume that's already doing a lot visually, a bold print under a sheer kaftan with its own pattern just looks busy and a bit much, not deliberate. Plain or lightly textured cover ups sit best over a patterned costume, and a patterned cover up wants a plain costume underneath so one or the other gets to be the interesting thing. Heavy statement jewellery fights with the loose, floaty shape too, chunky necklaces catch on open weave fabric and pull threads, so simple earrings are the safer choice. Hold any sheer piece up to a window in the shop before buying, not just under the shop lighting, because that's where you'll actually see how much it shows.
Where it falls short is as actual protection. A cover up is not sun cream and it's not something to rely on for coverage if you burn easily, a proper layer with built in UPF does that job properly and a cotton gauze throw over your shoulders does not.