A good bikini, taken care of correctly, should last two or three summers. Most don't make it through one because the care gets skipped at the wrong moments.
Rinse the suit in cold water after every use. Salt, sunscreen, and chlorine all break down elastane over time, and the damage is cumulative — the first few sessions feel fine and the suit starts to stretch out two months later. Rinsing takes twenty seconds. It's the single most useful thing you can do.
Hand wash rather than machine wash. Even a gentle cycle has enough agitation and spin to deform the fabric. A little gentle detergent, lukewarm water, your hands. Rinse until the water runs clear.
Dry the suit flat, in the shade. Wringing breaks the elastic fibers. Laying it flat on a towel is better than hanging — hanging pulls the weight of the wet fabric through the straps. Direct sun on a wet suit fades the color faster than almost anything else.
Two things that damage bikinis faster than people expect: sitting on rough surfaces in the suit — pool ledges, stone tiles, wooden decks — and applying sunscreen directly onto the fabric before it's absorbed into your skin. Both degrade the material in ways that don't show immediately.
Chlorine accumulates regardless of how carefully you wash. If you're swimming in a pool multiple days a week, keeping a separate pool suit makes sense. The beach suit stays for the beach and lasts much longer.
Melancia fabric is 82% polyamide, 18% elastane — built to handle saltwater and sun. The above routine still applies. The fabric handles the beach; you handle the care.