I'm not a runner, but I recall back in the day (circa late 1980s-early 1990s), seeing the Ironman triathalon on TV. All the male competitors wore briefs all the way through the competition, adding a tank top and shoes for the cycling and running portions

I've run the length of Kaanapali Beach on Maui a few times on vacation. Barefoot works on the wet sand near the water's edge.

I normally package myself down wearing a Speedo, but found that for running, that really crowded things so now I just pull everything up before I go.