Most people buy swimwear for the one day they'll actually wear it, the holiday, the hen do, the hotel pool with the slide, and then stand in a changing room lit like an interrogation room making a decision they'll live with for three summers. The mistake isn't the lighting, it's buying for that single moment rather than for the body you'll have on a Tuesday in your own bathroom, bending to pick up a towel, sitting down on a lounger, actually existing in it.
A decent piece earns its keep across more than one trip if you treat it properly. Rinse it in cold water straight after it's been in chlorine or salt, every time, because both will eat elastane if you let them sit, and that's when a suit goes baggy at the bum or a bikini top loses its grip across the back. Never wring it, never tumble dry, never leave it balled up damp in a bag for two days, which I have done and watched the colour go patchy as a result. Looked after, a good one holds its shape through a whole season of actual swimming, not just floating. Looked after badly, you get six weeks and a saggy seat.
The difference between a good one and a sad one is mostly in things you can't see from across the shop floor. Pull the fabric, a proper matte swim fabric has some weight and bounce back to it, the cheap shiny kind goes thin and see through within a month, especially when wet, which is its own kind of horror. Check the lining, it should sit separate from the outer, not stuck to it, or you'll get see through patches exactly where you don't want them. Look at the stitching round the leg and underarm, it should be flat and tucked, not a ridge that digs in the second you sit down. Clasps and rings should feel sturdy, not like something off a cheap handbag.
If you've got nothing decent in the drawer, start with a well cut one piece in black or navy, something with proper support across the back and no gaping at the front when you lean forward, because that one fit test tells you more than any amount of print or detail. It pairs with whatever you'd throw over anything else, a linen shirt, denim shorts, a long cardigan for the walk home, nothing fussy required.
Reach for it the moment there's water involved, a pool, a lido, a beach, a bath that got out of hand, and let the rest of the wardrobe catch up around it.