Swimming laps in chlorinated pool? Afterward Chloroban / polyester lasts longest, but is not form-fitting which swimmers desire when racing competitively. For waterpolo we wear polyester (most permanent) and we wear 2 layers because adversaries catch and tear at everything. Waterpolo is quite aggressive sport - Refs can not see what occurs submerged. Plenty of dunking/practically drowning poorer players.
But I digress, About the cloths: If merely lounging round the pool / beach and much more worried about looks than function? Unless its the loose mens board shorts fashion which is 100% polyester. Cotton is making a little recovery more recently in girls suits but it could bunch up and not stay in place together with it does with girls swimwear 10 -to 20% stretchy cloth (lycra/chloroban/spandex) to help it stay put. Additionally cotton does not dry out within minutes after like the polyester, nylon, lycra, elastane does. My buddies make fun of my 1 friend that wears his cotton boxers under his board shorts cuz once you escape the water you are stuck with wet tail/crotch the remainder of the girls swimwear day GROSSS! I really don't believe cotton or natural fibers make a great swimming fabric.