First thing first: you won't find any cotton-heavy fabric in board shorts since it will get too heavy when soaked with water and thus weigh you down. Lining: Swim trunks almost always have mesh linings to provide more support. Boards shorts, on the other hand, do not usually have linings; in fact, surfers and sailors avoid liners because that extra fabric can cause abrasion against the skin. Instead, board shorts offer a free, less restrictive fit that helps air circulate to keep you cool on those hot summer days, and give you more wiggle room and space for movement on the waves. Really, if you're performing heavy activities in water, no lining is the way to go.

JustSun Mens Swim Shorts with Zip Pockets Mesh Lining Quick

From fit, lining, waistband, fabric, and occasion, board shorts actually differ from swim trunks in many ways. Length: The main difference here is the length. While swim trunks generally hit at the thigh, men's board shorts tend to be longer, usually ending right at the knee or slightly below it. This helps provide more coverage in general, and it protects you from the sun. Traditional board shorts tend to be baggier in shape and are known for being less restrictive—one of the many reasons they are recommended for highly intense activities, such as surfing, wakeboarding, and beach games.