My buddy and I (late 20s at the time) went on a hike/camp trip. After a day of hiking we set up camp near a river. That evening we decided to go for a swim to cool and clean off. We had brought bathing suits but to avoid having something wet to dry we decided to skip the suits and just skinny dip. We were both straight but had no issue having seen each other in the gym locker room before.

We went in far enough to go under the water but eventually hung out near the side. It was only a couple feet deep but we stayed immersed up to our necks.

At one point I realized how good the rushing water felt. At some point I joked “I feel like the river is trying to give me a handjob”. My buddy laughs, stands up and he is hard as a rock. Apparently he was feeling it too.