Beach Bunny has a 45-day limit to take returns of unworn/unwashed items. Come past that timeframe and you won’t be able to return the item, much less get a refund for it. Similarly, PacSun and Zaful accept returns only if the swimwear is in its original condition, and you bring it back within 30 days from the purchase.
Costco, where there’s a variety of swimwear, has no issue taking back returned swimwear regardless of the time of purchase or how much you’ve worn it. Maybe the furnace where each store keeps in the back to burn compromised returns has something to do with it.