When I was in elementary school I regularly "forgot" my swimwear and borrowed friends. They usually felt more baggy and conservative compared to my own suits. I was too young to get boners but enjoyed the range of styles and fabrics while we were young and mostly uninhibited.
As I grew older and realised my fetish, I took as many opportunities as I could to secretly find my friends swimwear and try them on. When I was horny like this I leaked precum like crazy and after a while would soak the suit.

I would get huge boners, and even blew a hot load in my cousins green speedo by mistake. I wiped it up with tissue and put it at the bottom of the drawer, hoping he wouldn't notice.

A flatmate had a very old Adidas brief, perhaps a boys size or very small mens. He was about 25 and I doubt if he wore it any more, but he was hot and I often held it and imagined his hairy little cock nestled in the pouch. I tried it on but with a piece of folded toilet paper between my tip and the fabric to avoid any awkward staining and explaining.

I worked alone (security) at an area high school at night on weekends. Once, walking by the swim coach's office, I noticed that the guys on the swim team left their speedos hanging on hooks in his office. I found one that was my size, put it on and went for a swim. It fit perfectly, and I loved the way it felt. It was a light blue lycra suit. I "borrowed" the team speedos several more times. I wish I knew what its owner looked like in them.