Black Sunga, Brazilian swim trunks
The sunga is a true classic in Brazil and very different from the much smaller "Speedos". This far more flattering Brazilian style is much wider at the sides and is fully lined for extra comfort, has a slightly padded look and a looser fitting that makes it excellently suited for any beach sport activity. A lot of men in Brazil wear them underneath their boardshorts for surfing or for running on the beach.
Available in M and L. Guys don't need a size chart, right? The model in the image is wearing an M.